remove _XOPEN_SOURCE in code

lustre, or specifically the header files lustre brings in (quota.h) show
a problem with caddr_t not being defined if XOPEN_SOURCE is set.  We
provided this define so we could make use of pread/pwrite, but instead
we will use our replacement pread/pwrite if one does not exist.

See #1973
Signed-off-by: Ken Raffenetti <raffenet@mcs.anl.gov>

src/mpi/romio/adio/ad_lustre/ad_lustre_rwcontig.c

@@ -8,11 +8,6 @@
  *   Copyright (C) 2008 Sun Microsystems, Lustre group
  */
 
-#ifdef _STDC_C99
-#define _XOPEN_SOURCE 600
-#else
-#define _XOPEN_SOURCE 500
-#endif
 #include <unistd.h>
 
 #include <stdlib.h>

src/mpi/romio/adio/ad_xfs/ad_xfs.h

@@ -8,11 +8,6 @@
 #ifndef AD_XFS_INCLUDE
 #define AD_XFS_INCLUDE
 
-#ifdef _STDC_C99
-#define _XOPEN_SOURCE 600
-#else
-#define _XOPEN_SOURCE 500
-#endif
 #include <unistd.h>
 #include <sys/types.h>
 #include <fcntl.h>

src/mpi/romio/adio/common/ad_read.c

@@ -6,11 +6,6 @@
  */
 
 
-#ifdef _STDC_C99
-#define _XOPEN_SOURCE 600
-#else
-#define _XOPEN_SOURCE 500
-#endif
 #include <unistd.h>
 
 #include "adio.h"

src/mpi/romio/adio/common/ad_write.c

@@ -6,11 +6,6 @@
  */
 
 
-#ifdef _STDC_C99
-#define _XOPEN_SOURCE 600
-#else
-#define _XOPEN_SOURCE 500
-#endif
 #include <unistd.h>
 
 #include "adio.h"