configure: improve the error message if gbm missing

This improves the error message in the case where libgbm is missing when
the KMS egl platform has been enabled.

https://bugzilla.gnome.org/show_bug.cgi?id=706808

Reviewed-by: Neil Roberts <neil@linux.intel.com>
build: fix building introspection data

This fixes the build with --enable-introspection. I'm not sure why
g-ir-scanner seems to parse all public headers in isolation instead of
being able take a more limited list of top-level public headers and
automatically parse all necessary #include directives but this means we
have to special case how we define and undefine __COGL_H_INSIDE__ to
subvert the guards we have in place for detecting misuse of the headers.

Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit e0b2255876c1cf11d124d5ae37cbe9a6e43777f1)