zlib cannot be built out-of-tree, and previously, we were building in
the "src" directory, not in the "build" directory.  The
make_build_path() return value was unused.

It's disabled anyway.

Use wchar_t for everything on Windows.  Solves a lot of filesystem
charset problems.

Don't use FILE_ATTRIBUTE_NORMAL, it's a "magic" value for something
else.  To check if a file is a regular file, we need to check if it's
NOT a directory (or a device).