Enable building for Windows with Travis-CI

This PR enables building for Windows with Travis-CI. ctags has always been a multi-platform so building on Windows is important as well. Every now and then the Windows build is broken by Linux specific code and now Travis-CI warns you for that.

First it installs the mingw-w64 cross compiler and after that it it adds the build steps needed to cross compile ctags.

I could not find a way to only let these steps happen with gcc and not with clang. The second time is redundant. If somebody knows a way to fix that, please let know!