*** WARNING: THIS COMMIT CHANGES THE BUILD ***

Do not recurse into the backend directories to build private, internal
libraries.

We only recurse from clutter/ into the cogl sub-directory; from there,
we don't recurse any further. All the backend-specific code in Cogl and
Clutter is compiled conditionally depending on the macros defined by the
configure script.

We still recurse from the top-level directory into doc, clutter and
tests, because gtk-doc and tests do not deal nicely with non-recursive
layouts.

This change makes Clutter compile slightly faster, and cleans up the
build system, especially when dealing with introspection data.

Ideally, we also want to make Cogl part of the top-level build, so that
we can finally drop the sed trick to change the shared library from the
GIR before compiling it.

Currently disabled:

  ‣ OSX backend
  ‣ Fruity backend

Currently enabled but untested:

  ‣ EGL backend
  ‣ Windows backend