I am running Slax 5 (Which is based off of Slackware), and I am trying to compile GRUB, but when I run ./configure I get this:

checking build system type... i686-pc-linux-gnu
checking host system type... i686-pc-linux-gnu
checking target system type... i686-pc-linux-gnu
checking for cmp... cmp
checking for bison... bison
checking for a BSD-compatible install... /usr/bin/ginstall -c
checking for gawk... gawk
checking whether make sets $(MAKE)... yes
checking for ruby... no
checking for gcc... gcc
checking for C compiler default output file name... a.out
checking whether the C compiler works... yes
checking whether we are cross compiling... no
checking for suffix of executables...
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
checking for gcc option to accept ANSI C... none needed
checking for special C compiler options needed for large files... no
checking for _FILE_OFFSET_BITS value needed for large files... 64
checking for _LARGE_FILES value needed for large files... no
checking whether byte ordering is bigendian... no
checking how to run the C preprocessor... /lib/cpp
configure: error: C preprocessor "/lib/cpp" fails sanity check
See `config.log' for more details.

So, /lib/cpp is insane? That makes no sense

How can I get this working?

That error message is a general failure which means your system is not properly configured for compiling software. Unfortunately, there could be a number of reasons for that, which are only compounded by the fact that you are running a live CD rather than an installed and more thouroughly documented/used distribution.

Does SLAX even include a compiler? To see if GCC is even installed, run:

You could also try looking at config.log, which should be in the directory where you ran ./configure.

Not sure why you would EVER need to compile GRUB. Don't all distros now come with bootloaders?

In the repositories for my system (Mepis6.5), I find packages for GRUB, GRUB-II, and LILO.

The last time I used Slack, they only shipped LILO. I prefer GRUB to LILO to I always compiled GRUB from source.

GCC is installed, I downloaded a "Development" Slax module from the website and installed it (even though it's a live CD you can save changes) so I have GCC and GDB and all that, I've compiled a simple program I've written before. I'll take a look at config.log, I forgot to do that.