Merge pull request #6 from nh2/nh2fixes

Improve flags conflicting with -M section
Formatting.
Remove hack to append "-o a.out" when we actually don't want to build any executeble.

This behaviour
  * caused parmake / ghc to compile with only one thread in cetain
    situations
  * hid the bug that using only ghc -M and then ghc -c per module,
    parmake *cannot* detect whether an external (e.g. used package)
    module (interface file) changed when doing an incremental build
    (ghc itself can detect that).
    We will have to detect this by inspecting .hi files, like ghc does.
Skip mtime check if SKIP_TIME_CHECK env var is set