* configure.in: Rewrite check for GNU regex and the

* configure.in: Rewrite check for GNU regex and the
--without-included regex option, and move it into the "Checks for
library functions" section.  This makes us use the system regex
again by default on systems with version 2 of the GNU C library.
This was apparently broken.
* gdb_regex.h [!USE_INCLUDED_REGEX] (_REGEX_RE_COMP): Define.
* acconfig.h (USE_INCLUDED_REGEX): Remove.
* config.in, configure: Regenerated.