gapi.xsd: Reorder attribute definitions so that the name comes first

This improves the readibility of the schema, but does not really change
it.
glib: Use GTHREAD preprocessor flag with old versions of glib

We switch the logic from DISABLE_GTHREAD_CHECK to ENABLE_GTHREAD_INIT
to make the define clearer, and so that it is actually needed when using
older versions of glib, not newer.

Signed-off-by: Bertrand Lorentz <bertrand.lorentz@gmail.com>