movit: fix build

pkgs/by-name/mo/movit/package.nix

···
     libepoxy
   ];
 
-  env = lib.optionalAttrs stdenv.hostPlatform.isDarwin {
+  env = {
+    NIX_CFLAGS_COMPILE = "-std=c++17"; # needed for latest gtest
+  } // lib.optionalAttrs stdenv.hostPlatform.isDarwin {
     NIX_LDFLAGS = "-framework OpenGL";
   };