Fix the circular dependency by moving the generated
headers to gdk/version/, and build that directory
first.
Misc other fixes, such as putting the custom targets
as sources, not depedencies, and using the correct
major version in the generator script.
Let's poach the same script used by GLib to avoid having to add all the
version macros by hand every time we increment the GTK version.
This is a work in progress:
- need to rename the GLIB_STATIC_COMPILATION check
- circular dependency: libgtkcss depends on gdkversionmacros.h, but libgdk
depends on libgtkcss