private/windows.h: guard against _ANONYMOUS_UNION/STRUCT redefinition

It's already defined in Cygwin's /usr/include/w32api/_mingw.h

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

include/private/windows.h

@@ -1,20 +1,29 @@
 /*
  * Copyright © 2009 Université Bordeaux
- * Copyright © 2020 Inria.  All rights reserved.
+ * Copyright © 2020-2022 Inria.  All rights reserved.
  *
  * See COPYING in top-level directory.
  */
 
 #ifndef HWLOC_PRIVATE_WINDOWS_H
 #define HWLOC_PRIVATE_WINDOWS_H
 
+#ifndef _ANONYMOUS_UNION
 #ifdef __GNUC__
 #define _ANONYMOUS_UNION __extension__
-#define _ANONYMOUS_STRUCT __extension__
 #else
 #define _ANONYMOUS_UNION
+#endif /* __GNUC__ */
+#endif /* _ANONYMOUS_UNION */
+
+#ifndef _ANONYMOUS_STRUCT
+#ifdef __GNUC__
+#define _ANONYMOUS_STRUCT __extension__
+#else
 #define _ANONYMOUS_STRUCT
 #endif /* __GNUC__ */
+#endif /* _ANONYMOUS_STRUCT */
+
 #define DUMMYUNIONNAME
 #define DUMMYSTRUCTNAME