configure.ac: require GLib 2.32 (if enabled)

Since version 2.32, g_thread_init() is deprecated and a no-op.  Let's
upgrade that (optional) dependency so we can remove all those
g_thread_init() calls.

src/Main.cxx

@@ -437,11 +437,6 @@ int mpd_main(int argc, char *argv[])
 
 #ifdef HAVE_GLIB
 	g_set_application_name("Music Player Daemon");
-
-#if !GLIB_CHECK_VERSION(2,32,0)
-	/* enable GLib's thread safety code */
-	g_thread_init(nullptr);
-#endif
 #endif
 #endif