Disallow --classic for Windows

With all the config changes, mk1mf is broken on Windows.

Reviewed-by: Rich Salz <rsalz@openssl.org>

Configure

diff --git a/Configure b/Configure
index 87a46a9..0972cfe 100755
--- a/Configure
+++ b/Configure
@@ -901,7 +901,7 @@
         if $srcdir ne $blddir;
 
     $target{build_scheme} = { unix    => [ "unixmake" ],
-                              windows => [ "mk1mf", $target{build_scheme}->[2] ],
+                              windows => undef,
                               VMS     => undef } -> {$target{build_scheme}->[1]};
 
     die "Classic mode unavailable on this platform\n"