With MSVC, converting an empty macro value with STRINGIFY doesn't seem to
work. (Maybe it's producing nothing rather than the blank literal, ""?)
Work around it by allowing __none__ as a substitute for blank.
* Reorder ship targets to prefer MSYS, then MSYS2, then Cygwin
The MSYS build is much more likely to fail, both because its Cygwin
environment is poor (e.g. no C++11, no std::wstring) and because its
MinGW environment is poor (e.g. missing/broken SDDL APIs)
* Use PowerShell instead of the pefile module, because it's much faster,
even with the extra overhead of starting PowerShell.
* For MSYS, instead of disabling parallel make, use mingw32-make.exe.
Apparently, with this change, it's necessary to use forward slashes when
passing the PREFIX to make, so change that as well.
