Fixed MinGW warning.

src/renderer_d3d12.h

@@ -20,13 +20,14 @@
 
 #if defined(__MINGW32__) // BK - temp workaround for MinGW until I nuke d3dx12 usage.
 extern "C++" {
-#	if __cpp_constexpr >= 200704L && __cpp_inline_variables >= 201606L
+#	if defined(__cpp_constexpr)        && __cpp_constexpr        >= 200704L \
+	&& defined(__cpp_inline_variables) && __cpp_inline_variables >= 201606L
 	__extension__ template<typename Ty>
 	constexpr const GUID& __mingw_uuidof();
 #	else
 	__extension__ template<typename Ty>
 	const GUID& __mingw_uuidof();
-#	endif
+#	endif // __cpp_*
 
 	template<>
 	const GUID& __mingw_uuidof<ID3D12Device>()