accpet only posix-regex and use bundled gnu_regex if regcomp isp not available or it broken

This patch does two things.

1. Remove --with-posix-regex option

   Now ctags command line using --regex-<LANG> can be part of a
   optlib, we should use single regex syntax. So I removed the code
   utilizing other than posix-regex(regcomp).

2. Utilize bundled gnu_regex library if regcomp is not available on
   the target platform or it is available but is borken.

   Assumed target of this modification is mingw32 and mingw64.
   I guess regcomp is not available on these platforms.

TODO:

	* update f-windows.rst, and
	* dont' print --<LANG>-regex option if "regex" feature is not available

Signed-off-by: Masatake YAMATO <yamato@redhat.com>