[Cmake] Runtime search paths under Unix

[Amitha Perera]

I've patched... sorry, enhanced... the unix Makefile generator to
add runtime search paths to the link line. The two variables it needs
are set in the cache, and initially generated from
Templates/configure.in. I used tcl8.3's configure.in to try to get the
appropriate values for as many platforms as I could, but I can only
guarantee it for FreeBSD.

What is missing is a clean way to specify additional runtime search
paths (for system specific libraries), since variables such as
CMAKE_SHLIB_LINK are used only when linking a library, not when
linking an executable. Perhaps yet another variable? CMAKE_EXE_LINK?

Amitha.