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