Merge pull request #34 from bsilver8192/fix-macro-redefinition

Fix redefinition of _XOPEN_SOURCE.

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>

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>

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;