Check gthread-2.0 to support RHEL6: rel to #21

  * On glib <2.32, main() calls g_thread_init but doesn't link such
    symbol due to configure doesn't care of gthread-2.0.
Relax configure requirements of autoconf; rel to #21
Workaround for GTK < 2.20
Merge 'support/el6' and close #21