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/cmdline.c

@@ -77,10 +77,8 @@ static void version(void)
 	exit(EXIT_SUCCESS);
 }
 
-#if GLIB_CHECK_VERSION(2,12,0)
 static const char *summary =
 	"Music Player Daemon - a daemon for playing music.";
-#endif
 
 bool
 parse_cmdline(int argc, char **argv, struct options *options,
@@ -118,9 +116,7 @@ parse_cmdline(int argc, char **argv, struct options *options,
 	context = g_option_context_new("[path/to/mpd.conf]");
 	g_option_context_add_main_entries(context, entries, NULL);
 
-#if GLIB_CHECK_VERSION(2,12,0)
 	g_option_context_set_summary(context, summary);
-#endif
 
 	ret = g_option_context_parse(context, &argc, &argv, &error);
 	g_option_context_free(context);