diff --git a/src/base/mutex.h b/src/base/mutex.h
index 37527d5..ced2b99 100644
--- a/src/base/mutex.h
+++ b/src/base/mutex.h
@@ -134,7 +134,9 @@
   // *does* cause problems for FreeBSD, or MacOSX, but isn't needed
   // for locking there.)
 # ifdef __linux__
-#   define _XOPEN_SOURCE 500  // may be needed to get the rwlock calls
+#   ifndef _XOPEN_SOURCE  // Some other header might have already set it for us.
+#     define _XOPEN_SOURCE 500  // may be needed to get the rwlock calls
+#   endif
 # endif
 # include <pthread.h>
   typedef pthread_rwlock_t MutexType;