more potential windows macro redefinition

src/google/protobuf/compiler/subprocess.h

@@ -34,7 +34,9 @@
 #define GOOGLE_PROTOBUF_COMPILER_SUBPROCESS_H__
 
 #ifdef _WIN32
+#ifndef WIN32_LEAN_AND_MEAN
 #define WIN32_LEAN_AND_MEAN  // right...
+#endif
 #include <windows.h>
 #else  // _WIN32
 #include <sys/types.h>