are these warnings harmful for building LFS7.1 ?
 

in 5.7 section , when I run configure command , below result produced :

lfs@UBUNTU-32bit:/mnt/lfs/sources/glibc-2.14.1$ cd ../glibc-build
lfs@UBUNTU-32bit:/mnt/lfs/sources/glibc-build$ ../glibc-2.14.1/configure --prefix=/tools --host=$LFS_TGT --build=$(../glibc-2.14.1/scripts/config.guess) --disable-profile --enable-add-ons --enable-kernel=2.6.25 --with-headers=/tools/include libc_cv_forced_unwind=yes libc_cv_c_cleanup=yes
checking build system type... i686-pc-linux-gnu
checking host system type... i686-lfs-linux-gnu
checking for i686-lfs-linux-gnu-gcc... i686-lfs-linux-gnu-gcc
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether i686-lfs-linux-gnu-gcc accepts -g... yes
checking for i686-lfs-linux-gnu-gcc option to accept ISO C89... unsupported
checking for gcc... gcc
checking how to run the C preprocessor... i686-lfs-linux-gnu-gcc -E
checking for i686-lfs-linux-gnu-g++... no
checking for i686-lfs-linux-gnu-c++... no
checking for i686-lfs-linux-gnu-gpp... no
checking for i686-lfs-linux-gnu-aCC... no
checking for i686-lfs-linux-gnu-CC... no
checking for i686-lfs-linux-gnu-cxx... no
checking for i686-lfs-linux-gnu-cc++... no
checking for i686-lfs-linux-gnu-cl.exe... no
checking for i686-lfs-linux-gnu-FCC... no
checking for i686-lfs-linux-gnu-KCC... no
checking for i686-lfs-linux-gnu-RCC... no
checking for i686-lfs-linux-gnu-xlC_r... no
checking for i686-lfs-linux-gnu-xlC... no
checking for g++... g++
configure: WARNING: using cross tools not prefixed with host triplet
checking whether we are using the GNU C++ compiler... yes
checking whether g++ accepts -g... yes
configure: running configure fragment for add-on libidn
configure: running configure fragment for add-on nptl
checking for assembler gnu_indirect_function symbol type support... yes
checking whether .text pseudo-op must be used... yes
checking for assembler global-symbol directive... .globl
checking for assembler .type directive prefix... @
checking sysdep dirs... sysdeps/i386/elf nptl/sysdeps/unix/sysv/linux/i386/i686 sysdeps/unix/sysv/linux/i386/i686 nptl/sysdeps/unix/sysv/linux/i386 sysdeps/unix/sysv/linux/i386 nptl/sysdeps/unix/sysv/linux nptl/sysdeps/pthread sysdeps/pthread sysdeps/unix/sysv/linux sysdeps/gnu sysdeps/unix/common sysdeps/unix/mman sysdeps/unix/inet sysdeps/unix/sysv/i386 nptl/sysdeps/unix/sysv sysdeps/unix/sysv sysdeps/unix/i386 nptl/sysdeps/unix sysdeps/unix sysdeps/posix sysdeps/i386/i686/fpu sysdeps/i386/i686/multiarch nptl/sysdeps/i386/i686 sysdeps/i386/i686 sysdeps/i386/i486 nptl/sysdeps/i386/i486 sysdeps/i386/fpu nptl/sysdeps/i386 sysdeps/i386 sysdeps/wordsize-32 sysdeps/ieee754/ldbl-96 sysdeps/ieee754/dbl-64 sysdeps/ieee754/flt-32 sysdeps/ieee754 sysdeps/generic/elf sysdeps/generic
checking for a BSD-compatible install... /usr/bin/install -c
checking whether ln -s works... yes
checking whether /mnt/lfs/tools/bin/../lib/gcc/i686-lfs-linux-gnu/4.6.2/../../../../i686-lfs-linux-gnu/bin/as is GNU as... yes
checking whether /mnt/lfs/tools/bin/../lib/gcc/i686-lfs-linux-gnu/4.6.2/../../../../i686-lfs-linux-gnu/bin/ld is GNU ld... yes
checking for /mnt/lfs/tools/bin/../lib/gcc/i686-lfs-linux-gnu/4.6.2/../../../../i686-lfs-linux-gnu/bin/as... /mnt/lfs/tools/bin/../lib/gcc/i686-lfs-linux-gnu/4.6.2/../../../../i686-lfs-linux-gnu/bin/as
checking version of /mnt/lfs/tools/bin/../lib/gcc/i686-lfs-linux-gnu/4.6.2/../../../../i686-lfs-linux-gnu/bin/as... 2.22, ok
checking for /mnt/lfs/tools/bin/../lib/gcc/i686-lfs-linux-gnu/4.6.2/../../../../i686-lfs-linux-gnu/bin/ld... /mnt/lfs/tools/bin/../lib/gcc/i686-lfs-linux-gnu/4.6.2/../../../../i686-lfs-linux-gnu/bin/ld
checking version of /mnt/lfs/tools/bin/../lib/gcc/i686-lfs-linux-gnu/4.6.2/../../../../i686-lfs-linux-gnu/bin/ld... 2.22, ok
checking for pwd... /bin/pwd
checking for i686-lfs-linux-gnu-gcc... (cached) i686-lfs-linux-gnu-gcc
checking version of i686-lfs-linux-gnu-gcc... 4.6.2, ok
checking for gnumake... no
checking for gmake... no
checking for make... make
checking version of make... 3.81, ok
checking for gnumsgfmt... no
checking for gmsgfmt... no
checking for msgfmt... msgfmt
checking version of msgfmt... 0.18.1, ok
checking for makeinfo... makeinfo
checking version of makeinfo... 4.13, ok
checking for sed... sed
checking version of sed... 4.2.1, ok
checking for autoconf... no
configure: WARNING:
*** These auxiliary programs are missing or incompatible versions: autoconf
*** some features will be disabled.
*** Check the INSTALL file for required versions.
checking LD_LIBRARY_PATH variable... ok
checking whether GCC supports -static-libgcc... -static-libgcc
checking for bash... /bin/bash
checking for gawk... gawk
checking for perl... /usr/bin/perl
checking for install-info... /usr/bin/install-info
checking for bison... /usr/bin/bison
checking for signed size_t type... no
checking for libc-friendly stddef.h... yes
checking whether we need to use -P to assemble .S files... no
checking for .set assembler directive... yes
checking for assembler gnu_unique_object symbol type... yes
checking for .symver assembler directive... yes
checking for ld --version-script... yes
checking for .previous assembler directive... yes
checking for .protected and .hidden assembler directive... yes
checking whether __attribute__((visibility())) is supported... yes
checking for broken __attribute__((visibility()))... no
checking for broken __attribute__((alias()))... no
checking whether to put _rtld_local into .sdata section... no
checking for .preinit_array/.init_array/.fini_array support... yes
checking for libunwind-support in compiler... no
checking for -z nodelete option... yes
checking for -z nodlopen option... yes
checking for -z initfirst option... yes
checking for -z relro option... yes
checking for -Bgroup option... yes
checking for libgcc_s suffix...
checking for --as-needed option... no
checking whether --noexecstack is desirable for .S files... yes
checking for -z combreloc... yes
checking for -z execstack... yes
checking for -fpie... no
checking for --hash-style option... yes
checking for -fno-toplevel-reorder -fno-section-anchors... yes
checking for -fstack-protector... no
checking for -fgnu89-inline... yes
checking whether cc puts quotes around section names... no
checking for assembler .weak directive... yes
checking whether CFI directives are supported... yes
checking for ld --no-whole-archive... yes
checking for gcc -fexceptions... yes
checking for __builtin_expect... yes
checking for __builtin_memset... yes
checking for redirection of built-in functions... yes
checking for __thread... yes
checking for tls_model attribute... yes
checking for libgd... no
checking for is_selinux_enabled in -lselinux... no
checking for grep that handles long lines and -e... /bin/grep
checking for egrep... /bin/grep -E
checking for ANSI C header files... no
checking for sys/types.h... no
checking for sys/stat.h... no
checking for stdlib.h... no
checking for string.h... no
checking for memory.h... no
checking for strings.h... no
checking for inttypes.h... no
checking for stdint.h... no
checking for unistd.h... no
checking size of long double... 0
running configure fragment for sysdeps/i386/elf
checking for i386 TLS support... yes
running configure fragment for nptl/sysdeps/pthread
checking for forced unwind support... (cached) yes
checking for C cleanup handling... (cached) yes
running configure fragment for sysdeps/pthread
running configure fragment for sysdeps/unix/sysv/linux
checking for grep that handles long lines and -e... (cached) /bin/grep
checking for egrep... (cached) /bin/grep -E
checking installed Linux kernel header files... 2.0.10 or later
checking for kernel header at least 2.6.25... ok
running configure fragment for sysdeps/i386
checking for grep that handles long lines and -e... (cached) /bin/grep
checking for egrep... (cached) /bin/grep -E
checking for ANSI C header files... (cached) no
checking for sys/types.h... (cached) no
checking for sys/stat.h... (cached) no
checking for stdlib.h... (cached) no
checking for string.h... (cached) no
checking for memory.h... (cached) no
checking for strings.h... (cached) no
checking for inttypes.h... (cached) no
checking for stdint.h... (cached) no
checking for unistd.h... (cached) no
checking for cpuid.h... yes
checking if -g produces usable source locations for assembler-with-cpp... yes
checking for SSE4 support... yes
checking for assembler -mtune=i686 support... yes
checking for AVX support... yes
checking for -mno-vzeroupper support... yes
checking for old glibc 2.0.x headers... no
checking whether -fPIC is default... no
configure: creating ./config.status
config.status: creating config.make
config.status: creating Makefile
config.status: creating config.h
config.status: executing default commands
lfs@UBUNTU-32bit:/mnt/lfs/sources/glibc-build$

are these warnings harmful for building LFS7.1 ?

Nope, these are not harmful in this case.

In general: If the configure/make steps keep running and don't end with an error, all is ok.

Warnings can show up and although you should check them, most of the time they are not harmful. Autoconf, for example, isn't needed to build a temporary environment (which is what you're doing in chapter 5). "Normally" you want this one to be present, but in this special case it isn't needed (autoconf also isn't mentioned in the Host System Requirements).

thank you for your quick guidance :)