I interpreted this as saying "We're programming to the 0x0501 version of the Win32 API" and my intent was to say "If you enable THREADSAFE then we're programming to the 0x0600 version of the Win32 API". As I said, maybe this isn't the right thing to do.

I put `#error` in thread-utils.h, but I guess I can also put an error here in the CMake file that blocks THREADSAFE if the Windows target platform is WinXP.