external libevent: fix for Cygwin

Fix from Marco Atzeri for building on Cygwin.

Signed-off-by: Marco Atzeri <marco.atzeri@gmail.com>
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

opal/mca/event/external/external.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011-2015 Cisco Systems, Inc.  All rights reserved.
+ * Copyright (c) 2011-2018 Cisco Systems, Inc.  All rights reserved
  * Copyright (c) 2013      Los Alamos National Security, LLC.  All rights reserved.
  * Copyright (c) 2015-2017 Intel, Inc. All rights reserved.
  * Copyright (c) 2015-2017 Research Organization for Information Science
@@ -22,6 +22,10 @@
 
 #include "opal_config.h"
 
+#if defined(__CYGWIN__) && defined(WIN32)
+#undef WIN32
+#endif
+
 #include "event.h"
 #include "event2/event.h"
 #include "event2/thread.h"