init glib langs on startup

works around a crash under heavy load, see

https://github.com/openslide/openslide/issues/161

ChangeLog

@@ -1,5 +1,6 @@
 25/4/15 started 8.0.1
 - fix some compiler warnings 
+- work around a glib bug that can cause segv under load
 
 11/2/15 started 8.0
 - remove old doc stuff, lots of doc improvements

libvips/iofuncs/init.c

@@ -277,6 +277,12 @@ vips_init( const char *argv0 )
 		g_thread_init( NULL );
 #endif 
 
+	/* This does an unsynchronised static hash table init on first call --
+	 * we have to make sure we do this single-threaded. See: 
+	 * https://github.com/openslide/openslide/issues/161
+	 */
+	(void) g_get_language_names(); 
+
 	if( !vips__global_lock )
 		vips__global_lock = vips_g_mutex_new();