Signal handler ucontext_t not filled out correctly on OS X
bz#341419

Before:

== 589 tests, 220 stderr failures, 15 stdout failures, 0 stderrB failures, 0 stdoutB failures, 30 post failures ==

After:

== 591 tests, 220 stderr failures, 15 stdout failures, 0 stderrB failures, 0 stdoutB failures, 30 post failures ==


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15296 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/auxprogs/valgrind-di-server.c b/auxprogs/valgrind-di-server.c
index bfcdd18..86c5882 100644
--- a/auxprogs/valgrind-di-server.c
+++ b/auxprogs/valgrind-di-server.c
@@ -89,7 +89,9 @@
 #include "pub_core_libcfile.h"      // For VG_CLO_DEFAULT_LOGPORT
 
 /* Needed to get a definition for pread() from unistd.h */
+#ifndef _XOPEN_SOURCE
 #define _XOPEN_SOURCE 500
+#endif
 
 #include <stdio.h>
 #include <unistd.h>