configure: Check the result of AM_PATH_GLIB_2_0

configure: Check the result of AM_PATH_GLIB_2_0

The AM_PATH_GLIB_2_0 doesn't automatically cause the configure script
to fail if the test fails. This wouldn't usually cause any problems
because we later check for the right glib version using
PKG_CHECK_MODULES directly. However AM_PATH_GLIB_2_0 is more thorough
when checking because it also tries to run a program against the
library to read the version. If the macro fails but the pkg-config
check passes then nothing will define GLIB_GENMARSHAL and the build
step will fail in a confusing way.

This adds a check for the result and gives an AC_MSG_ERROR if it
fails. The glib dependencies have been moved out of CLUTTER_DEPS to
AM_PATH_GLIB_2_0.

http://bugzilla.openedhand.com/show_bug.cgi?id=2127