Despit these macros being defined in sys/stat.h, they are undefined
when used in the source strmode.c, perhaps due to the shielding macro
S_IRUSR being defined before.

--- openbsd-compat/strmode.c	2015-10-02 01:46:21.000000000 +0200
+++ openbsd-compat/strmode.c	2015-10-02 08:38:43.954398930 +0200
@@ -37,6 +37,16 @@
 #include <sys/stat.h>
 #include <string.h>
 
+#if !defined(S_ISUID)
+#define	S_ISUID	04000
+#endif
+#if !defined(S_ISGID)
+#define	S_ISGID	02000
+#endif
+#if !defined(S_ISVTX)
+#define	S_ISVTX	01000
+#endif
+
 /* XXX mode should be mode_t */
 
 void