Fix a couple of issues with MBInstallHeaders on OSX

* Use an icky environmental state variable to avoid creating multiple
  targets for the symlink step (otherwise MBInstallHeaders can't be
  used multiple times)

* Fix for parsing of the name input if it's not a single directory.

* All of this is quite ugly, we should figure out a more SCons-like
  way to do it.
Merge remote-tracking branch 'origin/master' into qt5