Qt4BuildConfiguration: Fix extractSpecFromArguments with symlinked dirs

The canonical form of the -spec argument is a relative path if the
mkspec is a child of the mkspec dir. Since we resolve all symlinks for
the mkspec, we must do the same for the mkspec dir.

Task-number: QTCREATORBUG-7908
Change-Id: I24628c2d49ff7ad078caa6fd3896cb407d0d7906
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>