gtkglext: build and bundle built-in gtkglext

gtkglext: build and bundle built-in gtkglext

There is a chicken-and-egg problem, so the build must be done in two steps,

First build built-in libraries:

	cmake -G "Unix Makefiles" -S. -Bbuild -DBUILTIN_GTKGLEXT=ON -DBUNDLE_LIBRARIES=ON
	cmake --build build --parallel $(nproc) -- builtins

Then detect the built-in libraries and build netradiant and install the bundle as usual:

	cmake -S. -Bbuild
	cmake --build build --parallel $(nproc) install

There is no need to set cmake options on second step but cmake still has to be called again to detect the new files.