Fix 'collecting from unknown thread' abort in leak-finding mode

* include/private/gc_priv.h [GC_PTHREADS && !GC_WIN32_THREADS]
(GC_in_thread_creation): Move variable declaration from
pthread_support.h.
* misc.c [!DONT_USE_ATEXIT && GC_PTHREADS && !GC_WIN32_THREADS]
(GC_exit_check): Set GC_in_thread_creation to TRUE before GC_gcollect
call.

include/private/gc_priv.h

@@ -1951,6 +1951,13 @@ GC_INNER GC_bool GC_try_to_collect_inner(GC_stop_func f);
 #define GC_gcollect_inner() \
                 (void)GC_try_to_collect_inner(GC_never_stop_func)
 
+#if defined(GC_PTHREADS) && !defined(GC_WIN32_THREADS)
+  GC_EXTERN GC_bool GC_in_thread_creation;
+        /* We may currently be in thread creation or destruction.       */
+        /* Only set to TRUE while allocation lock is held.              */
+        /* When set, it is OK to run GC from unknown thread.            */
+#endif
+
 GC_EXTERN GC_bool GC_is_initialized; /* GC_init() has been run. */
 
 GC_INNER void GC_collect_a_little_inner(int n);

include/private/pthread_support.h

@@ -158,11 +158,6 @@ GC_EXTERN GC_bool GC_thr_initialized;
 
 GC_INNER GC_thread GC_lookup_thread(pthread_t id);
 
-GC_EXTERN GC_bool GC_in_thread_creation;
-        /* We may currently be in thread creation or destruction.       */
-        /* Only set to TRUE while allocation lock is held.              */
-        /* When set, it is OK to run GC from unknown thread.            */
-
 #ifdef NACL
   GC_EXTERN __thread GC_thread GC_nacl_gc_thread_self;
   GC_INNER void GC_nacl_initialize_gc_thread(void);

misc.c

@@ -760,7 +760,13 @@ GC_API int GC_CALL GC_is_init_called(void)
   STATIC void GC_exit_check(void)
   {
     if (GC_find_leak && !skip_gc_atexit) {
-      GC_gcollect();
+#     if defined(GC_PTHREADS) && !defined(GC_WIN32_THREADS)
+        GC_in_thread_creation = TRUE; /* OK to collect from unknown thread. */
+        GC_gcollect();
+        GC_in_thread_creation = FALSE;
+#     else
+        GC_gcollect();
+#     endif
     }
   }
 #endif