build: Generate an instealled config header

build: Generate an instealled config header

Clutter has some platform-specific API that is accessible only if the
right backend has been compiled in. Third party applications that wish
to be portable across backends might want to use defines and other
pre-processor tricks to determine header inclusion and API usage.

While Clutter has an internal set of symbols it can use, third party
applications don't have the luxury of being able to access the config.h
generated by Clutter's configure script.

For this reason, Clutter should install a configuration header with a
series of namespaced defines that can be picked up by applications and
other third party code.