Correctly handle Unicode paths on Windows

[polter-rnd]

Currently on Windows it's not possible to run tests using either mettle.exe or subprocess test runner if the path contains unicode characters.

It happens because CreateProcessA and GetModuleFileNameA are not Unicode-aware.

The right way is to use generic function prototypes and let the preprocessor choose -A or -W function version depending on the UNICODE definition as described here.

Also added UNICODE definition by default to build.bfg on Windows platform.

Fixes #55

[jimporter]

Thanks for noticing this (and sorry about the delays in responding). I think we'll need somewhat more extensive changes here though for everything to work nicely. For one thing, it would probably help to switch from std::string to std::filesystem::path where possible. That should keep us from having to use TCHAR in many places. I'll start working on that now.