toytoolkit: make all EGL code dependant on cairo-egl

toytoolkit: make all EGL code dependant on cairo-egl

configure.ac: The toytoolkit clients used to get libEGL linked to them
even if there was no cairo-egl. This is useless, and actually harmful on
platforms, where libEGL absolutely requires one of the GL ES libraries
to be linked in, too.

Look for EGL-related packages only for cairo-egl with toytoolkit.

window.c: protect all GL header includes with HAVE_CAIRO_EGL, since that
is the only case we can support EGL, GL, or GLESv2 at all. In the case
we do not have cairo-egl, add enough definitions to let us build the
stubs for EGL-related functions.

Remove some #ifdefs that were inside of the same #ifdef already.

These changes allow to build sorfware rendering toytoolkit clients
without any bits of EGL libs or headers.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>