Improve GLib probe against old GLib versions.

The glib_check_version function was not available in older versions
of glib2, so this is a bad choice for the link test.  Let's use
g_get_prgname instead, which is available at least as far back
as glib 2.0.0.

More importantly, with gcc these old glib versions use 'extern inline'
in header files and expect GNU inline semantics.  This generally does
not work with modern gcc, so add an additional configure test which
will detect this problem and work around it.