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/input/curl_input_plugin.c

@@ -150,11 +150,6 @@ buffer_free_callback(gpointer data, G_GNUC_UNUSED gpointer user_data)
 	g_free(data);
 }
 
-/* g_queue_clear() was introduced in GLib 2.14 */
-#if !GLIB_CHECK_VERSION(2,14,0)
-#define g_queue_clear(q) do { g_queue_free(q); q = g_queue_new(); } while (0)
-#endif
-
 /**
  * Frees the current "libcurl easy" handle, and everything associated
  * with it.

src/input/lastfm_input_plugin.c

@@ -55,18 +55,12 @@ lastfm_input_init(const struct config_param *param)
 	if (passwd == NULL || user == NULL)
 		return false;
 
-#if GLIB_CHECK_VERSION(2,16,0)
 	lastfm_data.user = g_uri_escape_string(user, NULL, false);
-#else
-	lastfm_data.user = g_strdup(user);
-#endif
 
-#if GLIB_CHECK_VERSION(2,16,0)
 	if (strlen(passwd) != 32)
 		lastfm_data.md5 = g_compute_checksum_for_string(G_CHECKSUM_MD5,
 						    passwd, strlen(passwd));
 	else
-#endif
 		lastfm_data.md5 = g_strdup(passwd);
 
 	return true;
@@ -364,23 +358,16 @@ lastfm_input_open(struct input_stream *is, const char *url)
 		return false;
 	}
 
-#if GLIB_CHECK_VERSION(2,16,0)
 	q = g_uri_escape_string(session, NULL, false);
 	g_free(session);
 	session = q;
-#endif
 
 	/* "adjust" last.fm radio */
 
 	if (strlen(url) > 9) {
 		char *escaped_url;
 
-#if GLIB_CHECK_VERSION(2,16,0)
 		escaped_url = g_uri_escape_string(url, NULL, false);
-#else
-		escaped_url = g_strdup(url);
-#endif
-
 		p = g_strconcat("http://ws.audioscrobbler.com/radio/adjust.php?"
 				"session=", session, "&url=", escaped_url, "&debug=0",
 				NULL);