[CMake] Build cmake child project from cmake parent project

[Dan Kegel]

FWIW, I tried making cmake projects that be used in two ways unchanged:
1) build installed libraries that other projects (not just cmake
projects) can use
2) be included in an overarching cmake project that knows about subprojects
but it did not turn out to be practical.  Maybe I just hadn't drunk enough
cmake kool-aid yet, and I still hate git submodules with a passion,
but I currently suspect that the right way to organize an overarching
build system is to be build-system agnostic, and the best way to
make sure you don't fail is to make it as dumb and simple as possible.

Hence when I needed an overall build system to build 50 open source
packages which were a mix of pure make, cmake, and automake, I wrote
a really, really simple one myself:
  https://github.com/Oblong/yobuild
It's dumb as nails, but it works.