Bump GLib requirement to 2.20

See http://gstreamer.freedesktop.org/wiki/ReleasePlanning/GLibRequirement

configure.ac

@@ -500,7 +500,7 @@ LIBS="$save_libs"
 dnl *** checks for dependency libraries ***
 
 dnl GLib
-GLIB_REQ=2.18
+GLIB_REQ=2.20
 
 AG_GST_GLIB_CHECK([$GLIB_REQ])
 

gst/gst.c

@@ -368,11 +368,7 @@ gst_init_get_option_group (void)
      * called gst_init() yet or initialised the threading system otherwise, we
      * better issue a warning here (since chances are high that the application
      * has already called other GLib functions such as g_option_context_new() */
-#if GLIB_CHECK_VERSION (2,20,0)
     if (!g_thread_get_initialized ()) {
-#else
-    if (!g_thread_supported ()) {
-#endif
       g_warning ("The GStreamer function gst_init_get_option_group() was\n"
           "\tcalled, but the GLib threading system has not been initialised\n"
           "\tyet, something that must happen before any other GLib function\n"
@@ -428,11 +424,7 @@ gst_init_check (int *argc, char **argv[], GError ** err)
 #endif
   gboolean res;
 
-#if GLIB_CHECK_VERSION (2,20,0)
   if (!g_thread_get_initialized ())
-#else
-  if (!g_thread_supported ())
-#endif
     g_thread_init (NULL);
 
   if (gst_initialized) {
@@ -577,11 +569,8 @@ init_pre (GOptionContext * context, GOptionGroup * group, gpointer data,
   g_type_init ();
 
   /* we need threading to be enabled right here */
-#if GLIB_CHECK_VERSION (2,20,0)
   g_assert (g_thread_get_initialized ());
-#else
-  g_assert (g_thread_supported ());
-#endif
+
   _gst_debug_init ();
 
 #ifdef ENABLE_NLS