libcxx: get gcc version with gcc --version instead of pacman

Getting it from pacman has a problem that I have gcc-multilib installed,
not gcc, so pacman -Q gcc fails.
libcxx: disable FORTIFY_SOURCE, because it breaks everything
libcxx-svn: find gcc version and include dirs at makepkg time
libcxx-svn: disable FORTIFY_SOURCE, which produces a broken libc++