Fix compilation on RHEL-4: If _GNU_SOURCE isn't defined, then neither is pthread_spinlock_t and compilation fails

git-svn-id: http://svn.fedorahosted.org/svn/corosync/trunk@2542 fd59a12c-fef9-0310-b244-a6a79926bd2f

include/corosync/hdb.h

@@ -36,6 +36,10 @@
 #ifndef HDB_H_DEFINED
 #define HDB_H_DEFINED
 
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
 #include <errno.h>
 #include <assert.h>
 #include <stdlib.h>