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>
diff --git a/Android.common.mk b/Android.common.mk
index e8b9006..1e9f040 100644
--- a/Android.common.mk
+++ b/Android.common.mk
@@ -47,7 +47,9 @@
 ifeq ($(strip $(MESA_ENABLE_ASM)),true)
 ifeq ($(TARGET_ARCH),x86)
 LOCAL_CFLAGS += \
-	-DUSE_X86_ASM
+	-DUSE_X86_ASM \
+	-DHAVE_DLOPEN \
+
 endif
 endif