[CMake] Building third party libraries along with normal targets

[Robert Dailey]

Interested in hearing everyone's thoughts on this idea of mine.

Right now I have several third party libraries: openssl, boost,
libpng, zlib, etc. List goes on. I need to support these libraries on
at least 3 different platforms: ARM android, x86 linux, x86 windows.
It's a real pain in the rear to build each of these libraries a total
of 3 times (once per platform). It makes upgrades painful.

So I was thinking: If I set up each third party library to build from
source as a normal target with the rest of my targets, this would get
me the libraries for each third party lib "for free". Basically, they
would all build the same way no matter what platform I'm on. I can
move to other platforms or architectures in the future with no extra
effort.

The downside, of course, is that they build along with my normal
targets. Which means doing "ninja clean" will clean them, when I
really don't need to, and it will make build times get longer.

Any thoughts on this idea? Are there any other downsides? Is this the
best way to have library support across platforms?