Instead of adding -DCOGL_ENABLE_EXPERIMENTAL_2_0_API to the
cogl-2.0-experimental.pc file we now install a cogl2-experimental.h
that #defines COGL_ENABLE_EXPERIMENTAL_2_0_API before including
cogl.h.
The problem with having the define in the .pc file is that you might
develop a library that depends on the experimental 2.0 api internally
and then you might want to use that library with Clutter which still
uses the 1.0 API but the .pc file for your library will indirectly,
automatically enable the 2.0 api which can cause conflicts.
