Only redefine O_NONBLOCK when a valid alias exists

On mingw, O_NONBLOCK is not defined.  Since we were always
redefining to an invalid FNDELAY, the compile failure message
would mislead you during debugging.  Its better for compile
failure to point to real issue; which is missing O_NONBLOCK.

include/sane/sanei_backend.h

@@ -42,7 +42,9 @@
 # ifdef O_NDELAY
 #  define O_NONBLOCK O_NDELAY
 # else
-#  define O_NONBLOCK FNDELAY    /* last resort */
+#  ifdef FNDELAY
+#   define O_NONBLOCK FNDELAY    /* last resort */
+#  endif
 # endif
 #endif
 #endif /* HAVE_OS2_H */