regex: allow inclusion of <regex.h> before <limits.h>

regex: allow inclusion of <regex.h> before <limits.h>

Without this patch, portable programs had to include <limits.h> before
<regex.h> if they wanted a consistent value for RE_DUP_MAX.
I ran into this problem with a test version of GNU grep on Solaris 8.
* lib/regex.h: Include <limits.h> if _REGEX_INCLUDE_LIMITS_H.
This is done conditionally so that this change can be merged
back to glibc.
* m4/regex.m4 (gl_REGEX): Define _REGEX_INCLUDE_LIMITS_H if
using the included regex.