• Use the public COGL_HAS_GLES[12] define instead of the HAVE_COGL_*
ones which are private and defined in config.h,
• Install clutter-egl-headers.h which is needed by clutter-egl.h,
• Remove clutter-stage.h as it's uneeded and does not work since the
single clutter.h include policy,
• Install the egl headers into their own egl directory as the x11 and
glx backends do. The include should then be <clutter/egl/clutter-egl.h>,
so document it. It does not really break anything as nobody could
have used those broken headers.
