Fix MS-Windows compilation when OUTPUT_SYNC is not defined.

main.c (prepare_mutex_handle_string): Define conditioned on OUTPUT_SYNC.

ChangeLog

@@ -1,5 +1,8 @@
 2013-05-17  Eli Zaretskii  <eliz@gnu.org>
 
+	* main.c (prepare_mutex_handle_string): Define conditioned on
+	OUTPUT_SYNC.
+
 	* build_w32.bat: Copy config.h.W32 to config.h regardless of
 	whether or not we are building from SCM.
 

main.c

@@ -752,6 +752,8 @@ decode_output_sync_flags (void)
 
 #ifdef WINDOWS32
 
+#ifdef OUTPUT_SYNC
+
 /* This is called from start_job_command when it detects that
    output_sync option is in effect.  The handle to the synchronization
    mutex is passed, as a string, to sub-makes via the --sync-mutex
@@ -775,6 +777,8 @@ prepare_mutex_handle_string (sync_handle_t handle)
     }
 }
 
+#endif	/* OUTPUT_SYNC */
+
 /*
  * HANDLE runtime exceptions by avoiding a requestor on the GUI. Capture
  * exception and print it to stderr instead.