(Note from Evan: this is landing Scott's code more or less verbatim without a lot of analysis; it could maybe be simplified and reduced, but it's only intended to be used in the MSVC helper so it's fine to be experimental.)
This will be needed for performant builds on Windows.
Disable the size_t truncation warning. (Note that this leaves on the other truncation-related warnings, like int->char.)
This can be disabled by having -fno-color-diagnostics in CFLAGS, or setting TERM to dumb.