[CMake] Mixing CPack and ExternalProject

[Steven Stallion]

All,

I currently have a set of packages defined as ExternalProjects and
packaged up with CPack. I've noticed recently that the preinstall step
has begun failing for a given package and after a little digging, I've
realized that I've likely been using CPack incorrectly for years.

So, rather than poke around at a bespoke set of list files and trying
to puzzle out how I can disable preinstall, I figured I'd ask a
different question:

What is the best method of generating a CPack package built from an
ExternalProject (using autotools) such that the resulting package is
only built once?

TIA,

Steve