ENH: New link line item ordering algorithm

This change introduces a new algorithm for link line construction.  The
order it computes always begins with the exact link line specified by
the user.  Dependencies of items specified by the user are tracked, and
those that are not already satisified by the line are appended to it at
the end with minimal repeats.  This restores the behavior of CMake 2.4
and below while still fixing some of its bugs.  See issue #7546.