Recognize also Interix

I did some initial testing with building under Interix some years
ago. Nothing came out of it, and LibreOffice certainly won't build
with it. But just commit this triviality now to get rid of
win32-interix.diff in the build repo.

configure.in

@@ -1435,7 +1435,7 @@ case "$host_os" in
         test_cups=no
         _os=GNU
         ;;
-    cygwin*) # Windows
+    cygwin*|interix*) # Windows
         test_cups=no
         test_randr=no
         test_freetype=no