configure.ac: require GLib 2.16

Accidently, MPD has been using several GLib 2.16 functions for a
while, and nobody noticed yet.  To simplify the code base, let's bump
the minimum GLib version for MPD to 2.16.  That version is old enough,
and it's reasonable to expect users to have it.

src/main.c

@@ -108,10 +108,8 @@ glue_mapper_init(void)
 	const char *music_dir, *playlist_dir;
 
 	music_dir = config_get_path(CONF_MUSIC_DIR);
-#if GLIB_CHECK_VERSION(2,14,0)
 	if (music_dir == NULL)
 		music_dir = g_get_user_special_dir(G_USER_DIRECTORY_MUSIC);
-#endif
 
 	playlist_dir = config_get_path(CONF_PLAYLIST_DIR);