Avoid mempcpy in the regex code, as the string.h mempcpy stuff

Avoid mempcpy in the regex code, as the string.h mempcpy stuff
is causing more trouble than it's curing.
* lib/regex_internal.h (__mempcpy): Remove.
* lib/regcomp.c (regerror): Rewrite to avoid the need for mempcpy
(and make the code a tad smaller to boot).
* m4/regex.m4 (gl_PREREQ_REGEX): Don't check for mempcpy.