gtk3: defines for debugging, disable cast checks

Because we're using plain buildtype these have to be
passed manually.

See: https://gitlab.gnome.org/GNOME/gtk/blob/3.24.10/meson.build#L59

With autotools the mapping for the the options to the defines was:

yes:     G_ENABLE_DEBUG G_ENABLE_CONSISTENCY_CHECKS
minimum: G_ENABLE_DEBUG G_DISABLE_CAST_CHECKS
no:      G_DISABLE_CAST_CHECKS G_DISABLE_ASSERT G_DISABLE_CHECKS

So we're passing the exact ones that would've been used for minimum.

Additionally it isn't a good idea to pass the equivalents used for "no" 
as it eliminates G_ENABLE_DEBUG which disables pre-condition checks and 
assertions. The actual option only existed to serve people who needed a 
specific build of GTK for very specific environments. And now they are
much better served with meson's plain buildtype and figuring out what to
pass themselves.

pkgs/development/libraries/gtk+/3.x.nix

@@ -77,6 +77,13 @@ stdenv.mkDerivation rec {
     "-Dtests=false"
   ];
 
+  # These are the defines that'd you'd get with --enable-debug=minimum (default).
+  # See: https://developer.gnome.org/gtk3/stable/gtk-building.html#extra-configuration-options
+  NIX_CFLAGS_COMPILE = [
+    "-DG_ENABLE_DEBUG"
+    "-DG_DISABLE_CAST_CHECKS"
+  ];
+
   postPatch = ''
     files=(
       build-aux/meson/post-install.py