_XOPEN_SOURCE not needed for usleep() in Solarish systems.

@@ -12,11 +12,13 @@
  * express or implied. See the License for the specific language governing
  * permissions and limitations under the License.
  */
 
 /* Use usleep */
+#ifndef __sun
 #define _XOPEN_SOURCE 500
+#endif /* __sun doesn't need XOPEN_SOURCE defined for usleep() in unistd.h. */
 #include <unistd.h>
 
 #include <errno.h>
 #include <s2n.h>