pathchk: port to hosts where mbstate_t is replaced

pathchk: port to hosts where mbstate_t is replaced

This problem was discovered when trying to build git coreutils on
Solaris 8 sparcv9 with Sun C 5.8: the 3rd argument to mbrlen was
of type int * (a pointer to the gnulib replacement mbstate_t) but
the system mbrlen wants the system mbstate_t *.
* bootstrap.conf (gnulib_modules): Add mbrlen.
* src/pathchk.c (mbrlen, mbstate_t) [!(HAVE_MBRLEN && HAVE_MBSTATE_T)]:
Remove macros, which are wrong now that gnulib supplies
replacements for mbstate_t and mbrlen.