configure.ac: cleanup -> builds on more systems.

* removed forced '-all-static' for mingw-builds (now builds/works on win7 native mingw)
* cleaned up AC_SUBST calls and *FLAGS management (now builds (but not works, due to missing TLS support) on OpenBSD 5.6 w/ gcc-4.9
*  added C--link check for GMP in addition to the existing C++-link check.