gravit: fix build with SDL_compat 1.2.70

Refer to https://github.com/libsdl-org/sdl12-compat/commit/6595f6e405844cbca4a55e47eaa29ccec0cdd75f

pkgs/by-name/gr/gravit/package.nix

@@ -64,6 +64,10 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
 
+  NIX_CFLAGS_COMPILE = [
+    "-DSDL_INCLUDE_GLU_H"
+  ];
+
   meta = {
     broken = (stdenv.hostPlatform.isLinux && stdenv.hostPlatform.isAarch64);
     homepage = "https://github.com/gak/gravit";