Just to add my 2c to this problem and maybe help future travelers ....
I recently performed a 7.6 build and did not encounter this error. Using the exact same host configuration for a subsequent build of 7.7-rc1 the same errors came up (and led me to here)
Code:
/mnt/lfs/build-logs/5.23-gettext-make-src-msgfmt.log:73:plural-exp.c:20:29: fatal error: ../intl/pluralx.c: No such file or directory
/mnt/lfs/build-logs/5.23-gettext-make-src-msgfmt.log:77:make: *** [libgettextsrc_la-plural-exp.lo] Error 1
/mnt/lfs/build-logs/5.23-gettext-make-src-msgmerge.log:12:plural-exp.c:20:29: fatal error: ../intl/pluralx.c: No such file or directory
/mnt/lfs/build-logs/5.23-gettext-make-src-msgmerge.log:16:make: *** [libgettextsrc_la-plural-exp.lo] Error 1
/mnt/lfs/build-logs/5.23-gettext-make-src-xgettext.log:32:plural-exp.c:20:29: fatal error: ../intl/pluralx.c: No such file or directory
/mnt/lfs/build-logs/5.23-gettext-make-src-xgettext.log:36:make: *** [libgettextsrc_la-plural-exp.lo] Error 1
The fix was to add the 'make -C intl pluralx.c' just after 'make -C intl pluralx.c' described above.
There was one difference on my host configuration (which was a VM template), and that was I had to run install 'texinfo' to get binutils to compile. However the LFS book does not have texinfo as a required host package. I've noted both of these to the dev team.