diff --git a/AUTHORS b/AUTHORS
index 769d44a..3adbe3a 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -9,6 +9,7 @@
 # Please keep the list sorted.
 
 Abhishek Parmar <abhishek@orng.net>
+Brian Silverman <bsilver16384@gmail.com>
 Google Inc.
 Michael Tanner <michael@tannertaxpro.com>
 romange <romange@users.noreply.github.com>
diff --git a/CONTRIBUTORS b/CONTRIBUTORS
index b26e22b..df7c950 100644
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -23,6 +23,7 @@
 # Please keep the list sorted.
 
 Abhishek Parmar <abhishek@orng.net>
+Brian Silverman <bsilver16384@gmail.com>
 Fumitoshi Ukai <ukai@google.com>
 Håkan L. S. Younes <hyounes@google.com>
 Ivan Penkov <ivanpe@google.com>
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;