html-tidy: fix on mingw

This doesn't fix the cross-build by itself due to
https://github.com/NixOS/nixpkgs/issues/14965
But in this case one can hack around that "easily" by:
  (html-tidy.override {
    inherit (/*non-cross nixpkgs eval*/)
      cmake;
  }).crossDrv

pkgs/tools/text/html-tidy/default.nix

@@ -13,6 +13,9 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ cmake libxslt/*manpage*/ ];
 
+  cmakeFlags = stdenv.lib.optional
+    (stdenv.cross.libc or null == "msvcrt") "-DCMAKE_SYSTEM_NAME=Windows";
+
   # ATM bin/tidy is statically linked, as upstream provides no other option yet.
   # https://github.com/htacg/tidy-html5/issues/326#issuecomment-160322107