Fix Windows build (broken by PR #241).
- Restored now-missing PLATFORM_ROOT variable which was still in use
- Removed early return() in case of non-Windows. I want to keep this working for auxiliary files in case we need some later. Since there is no MingW directory, this should do nothing if that is the host system name.
Merge pull request #242 from simbody/fix_windows_build

Fix Windows build (broken by PR #241).