msxml3: complain louder if libxslt was missing at compile time

[Nikolay Sivov]

> +    ERR_(winediag)("libxslt headers were not found at compile time. This is not supported, expect problems!\n");
I'm not sure if it's supported or not. It's allowed to build msxml3 
without libxslt and some parts will work fine, but I can't imagine a 
real world situation when somebody really doesn't want to build with 
libxslt. In my opinion 'Expect problems.' (without exclamation mark) 
would be enough.