[Cmake] Include/link directory paths.

[Brad King]

Hello, all:

I have just checked in a replacement implementation for Ian's
std::set-based uniquification of include and link directories in their
respective paths.  The new version goes back to using vectors because path
ordering may be important in some projects.  If anyone writes code that
adds an include or link directory to a cmMakefile instance, please use
cmMakefile::AddIncludeDirectory or cmMakefile::AddLinkDirectory.  They
contain code to prevent a path from being added twice (n^2 behavior for
filling the include/link paths, but n will be small).

Thanks,
-Brad