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_stream.h

@@ -26,6 +26,10 @@
 #include <stdbool.h>
 #include <sys/types.h>
 
+#if !GLIB_CHECK_VERSION(2,14,0)
+typedef gint64 goffset;
+#endif
+
 struct input_stream {
 	/**
 	 * the plugin which implements this input stream