This simplifies the standalone CMakeLists.txt file arriving in the
next commit.
_WIN32 is defined by MVSC and the MinGW toolchain for all relevant
Windows versions.
We don't need to explicitly check for UNIX because it's the only
non-Windows case we care about.
This was a _ton_ of changes, made 15 years ago, so there are probably some
problems to work out still.
Among others: Engine/Base/Stream.* was mostly abandoned and will need to be
re-ported.
Still, this is a pretty good start, and probably holds a world record for
lines of changes or something. :)
