uxpty.c uses non-X/Open facilities (notably strsignal()), so don't define

_XOPEN_SOURCE.  We do still need _GNU_SOURCE in order to get grantpt()
on GNU systems.  This allows uxpty.c to compile on NetBSD.

[originally from svn r5675]

unix/uxpty.c

@@ -2,8 +2,6 @@
  * Pseudo-tty backend for pterm.
  */
 
-#define _XOPEN_SOURCE 600
-#define _XOPEN_SOURCE_EXTENDED
 #define _GNU_SOURCE
 
 #include <stdio.h>