gtk2: fix GNOME bug #557780

URL: https://gitlab.gnome.org/GNOME/gtk/issues/303

This patch will fix the assertion generated at startup for freeciv on
macOS. Conditionally applied on Darwin.

Gdk:ERROR:gdkimage-quartz.c:250:_gdk_image_new_for_depth: assertion failed: (depth == 24 || depth == 32)