Make threads mandatory

G_THREADS_ENABLED still exists, but is always defined. It is still
possible to use libglib without threads, but gobject (and everything
above it) is now guaranteed to be using threads (as, in fact, it was
before, since it was accidentally impossible to compile with
--disable-threads).

https://bugzilla.gnome.org/show_bug.cgi?id=616754

tests/asyncqueue-test.c

@@ -178,7 +178,6 @@ basic_tests (void)
 int 
 main (int argc, char *argv[])
 {
-#if defined(G_THREADS_ENABLED) && ! defined(G_THREADS_IMPL_NONE)
   gint   i;
   gint   max_threads = MAX_THREADS;
   gint   max_unused_threads = MAX_THREADS;
@@ -239,7 +238,6 @@ main (int argc, char *argv[])
 
   main_loop = g_main_loop_new (NULL, FALSE);
   g_main_loop_run (main_loop);
-#endif
 
   return EXIT_SUCCESS;
 }