Silence bothersome warning from -Wextra
Struct initializations such as those in `CLWrapper::Run` of file
`src/msvc_helper-win32.cc` causes MinGW GCC to spew warnings.
Give MinGW builds MSVC build helper superpowers
Note: _WIN32 is used instead of WIN32 to enable builds with MSVC
IDE, Windows SDK non-IDE command line tools, and mingw/mingw-w64