Update lib/regex from glibc via Gnulib

Update lib/regex from glibc via Gnulib

This syncs recent refactorings from glibc, and incorporates:
2018-10-15 libc-config: merge from glibc
2018-10-15 regex: depend on libc-config
* .gitignore: Do not ignore m4/_*.m4.
* lib/cdefs.h: New file, copied from Gnulib.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* lib/libc-config.h, m4/__inline.m4: New files, copied from Gnulib.
* lib/regcomp.c, lib/regex.c, lib/regex_internal.c:
* lib/regex_internal.h, lib/regexec.c:
Copy from glibc via Gnulib.