Don't abort if clang-format cannot be found

[CodeFinder2]

You can now still set CLANGFORMAT_FORCED to ON to abort (= old behavior).

I think it's more useful if it doesn't fail if clang-format is not installed and simply does not create the clangformat target.

Thanks for considering this! :-)

[nigels-com]

Looks good. We'd prefer our builds don't fail without clang-format too.