Windows build issues (#337 and #338)

Fixes both problems in this pull request. Even after the tests pass, should verify that:
- [x] Windows builds have `-m64` flag
- [x] a model (e.g. src/models/basic_estimators/normal_loc.stan) can be built without a file type in Windows.
- [x] a model can be built with `.exe` file type in Windows
- [x] a model can be built without a file type in non-Windows