Prevent a collision with the system's '#define errno ...' by guarding the variable declaration.

include/my_sys.h:
  There are platforms which use '#define errno ...' and then cause a collision with
  'extern int errno;', one example is OpenBSD.
  Rather than treat them explicitly by the 'HAVE_ERRNO_AS_DEFINE', we can check for these
  by using '#ifndef errno'.
  (Backported from 5.0, as the problem was discovered there first.)

include/my_sys.h

@@ -193,11 +193,13 @@ void   __CDECL hfree(void *ptr);
 #endif
 #endif /* MSDOS */
 
+#ifndef errno				/* did we already get it? */
 #ifdef HAVE_ERRNO_AS_DEFINE
 #include <errno.h>			/* errno is a define */
 #else
 extern int errno;			/* declare errno */
 #endif
+#endif					/* #ifndef errno */
 extern const char ** NEAR my_errmsg[];
 extern char NEAR errbuff[NRERRBUFFS][ERRMSGSIZE];
 extern char *home_dir;			/* Home directory for user */