[build2] Crash on Windows v0.6.1

[Boris Kolpackov]

Klaim - Joël Lamotte <mjklaim at gmail.com> writes:

> I just got back to this today and hit the same issue [...]

Ok, I've managed to reproduce and fix this, thanks for the report
and the test case!

The latest stage now issues this diagnostics for your test:

error: translation unit cxx{lol} is a module interface
  info: consider using mxx{} instead
  info: while applying rule cxx.compile to update obje{lol}
  info: while applying rule cxx.link to update exe{lol}
  info: while applying rule alias to update dir{./}
  info: failed to update dir{./}

Boris