Do not include gc_pthread_redirects.h from extra/gc.c unless pthreads

(refactoring)

* extra/gc.c [GC_NO_THREAD_REDIRECTS] (GC_PTHREAD_REDIRECTS_ONLY): Do
not define unless GC_PTHREADS.
* extra/gc.c [GC_NO_THREAD_REDIRECTS]: Do not include
gc_pthread_redirects.h unless GC_PTHREADS.

extra/gc.c

@@ -84,7 +84,7 @@
 /* pthread_stop_world.c, pthread_support.c or win32_threads.c). */
 /* This is only useful if directly included from application    */
 /* (instead of linking gc).                                     */
-#ifndef GC_NO_THREAD_REDIRECTS
+#if !defined(GC_NO_THREAD_REDIRECTS) && defined(GC_PTHREADS)
 #  define GC_PTHREAD_REDIRECTS_ONLY
 #  include "gc/gc_pthread_redirects.h"
 #endif