The first line of locale/gen-translit.pl is the following:

open F, "cat C-translit.h.in | gcc -E - |" || die "Cannot preprocess input file"

There are two problems with this:
1) This does not detect any errors while the shell command "cat C-translit.h.in
| gcc -E -" is running.  Instead, you get some obscure errors later on when
C-translit.h is using to compile something.
2) $(CPP) should be used instead of gcc -E

So my proposal would be to move this preprocessing to the Makefile and replace
gcc -E by $(CPP).