configure.ac: require GLib 2.32 (if enabled)

Since version 2.32, g_thread_init() is deprecated and a no-op.  Let's
upgrade that (optional) dependency so we can remove all those
g_thread_init() calls.

test/DumpDatabase.cxx

@@ -33,10 +33,6 @@
 #include "event/Loop.hxx"
 #include "util/Error.hxx"
 
-#ifdef HAVE_GLIB
-#include <glib.h>
-#endif
-
 #include <iostream>
 using std::cout;
 using std::cerr;
@@ -107,14 +103,6 @@ main(int argc, char **argv)
 		return EXIT_FAILURE;
 	}
 
-	/* initialize GLib */
-
-#ifdef HAVE_GLIB
-#if !GLIB_CHECK_VERSION(2,32,0)
-	g_thread_init(nullptr);
-#endif
-#endif
-
 	/* initialize MPD */
 
 	config_global_init();