Thanks for reaching out. As someone who has been working on integrating and using open source in embedded systems for the better part of two decades now, I have both experience and opinions on this.
By far and away, for moderate- to reasonably-complex packages, the build system infrastructure of choice—for better or worse—is GNU autotools. I rarely have run into CMake (perhaps on one or two occasions at most) for open source projects, particularly those for embedded systems.
I can generally take a GNU autotools-based package and integrate it into a large embedded systems project in 5-10 minutes. For other build systems, like CMake, Jam, custom configure scripts, custom makefile-based systems, etc. it's typically much longer.
It's worth pointing out that GNU autotools can be hard to use for the new and uninitiated. Nest / Google have tried to make this easier by creating:
to help templatize and commonize patterns. Nest / Google's OpenWeave
both use it to good effect.
I'd strongly advocate taking a look at GNU autotools and, in particular nlbuild-autotools for LwIP.