Fix build select with multiple folders on Windows

In Windows, when you had multiple folders open, this would fail to match
the folder the file currently open resides in, causing it to be unable
to detect a pre-existing project.xml, for instance.

Using the platform-agnostic os.sep fixed this issue.
Merge pull request #149 from Zshazz/zshazz_fix_sep

Fix build select with multiple folders on Windows