mesa/dlopen: use HAVE_DLOPEN instead of _GNU_SOURCE

mesa/dlopen: use HAVE_DLOPEN instead of _GNU_SOURCE

Patches changes mesa to use 'HAVE_DLOPEN' defined by configure and Android.mk
instead of _GNU_SOURCE for detecting dlopen capability. This makes dlopen to
work also on Android where _GNU_SOURCE is not defined.

[mattst88] v2: HAVE_DLOPEN is sufficient for including dlfcn.h, remove
	       mingw/blrts checks around dlfcn.h inclusion.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>