Some of the sources need to include <cogl/cogl-defines.h> which is in
the build directory. I think this directory gets added to the include
flags anyway by something so the actual building works but when
building the introspection data out of tree it was not included so it
failed to scan.
This removes all the remnants from being able to build Cogl standalone
while it was part of the Clutter repository. Now that Cogl has been
split out then standalone builds are the only option.
