need _XOPEN_SOURCE defined for pread.
*/
#if defined(HAVE_PREAD) && !defined(SOLARIS) && !defined(__OpenBSD__) && !defined(__NetBSD__) && !defined(__FreeBSD__) && !defined(TRU64)
-#ifdef _XOPEN_SOURCE
-#undef _XOPEN_SOURCE
-#endif
+#ifndef _XOPEN_SOURCE
#define _XOPEN_SOURCE 500
#endif
+#endif
#include <sys/types.h>
#include <sys/stat.h>