Define intptr_t in <unistd.h> as specified by POSIX

ok millert

src/include/unistd.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: unistd.h,v 1.83 2013/12/28 01:51:53 martynas Exp $ */
+/*	$OpenBSD: unistd.h,v 1.84 2014/04/01 05:04:50 matthew Exp $ */
 /*	$NetBSD: unistd.h,v 1.26.4.1 1996/05/28 02:31:51 mrg Exp $	*/
 
 /*-
@@ -325,6 +325,11 @@
 #endif
 #endif
 
+#ifndef	_INTPTR_T_DEFINED_
+#define	_INTPTR_T_DEFINED_
+typedef	__intptr_t		intptr_t;
+#endif
+
 __BEGIN_DECLS
 __dead void	 _exit(int);
 int	 access(const char *, int);