configure.ac: require GLib 2.12

Drop the required GLib version from 2.16 to 2.12, because many current
systems still don't have GLib 2.16.  This requires several new
compatibility functions in glib_compat.h.

src/input/curl_input_plugin.c

@@ -23,6 +23,7 @@
 #include "config.h"
 #include "tag.h"
 #include "icy_metadata.h"
+#include "glib_compat.h"
 
 #include <assert.h>