Fix redefinition of _XOPEN_SOURCE.

Under Debian Jessie, I get a warning about redefining _XOPEN_SOURCE
which is easy to fix.

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;