[CMake] How to build CMake so it works on an older Linux?

[Eric Wing]

I just discovered that CMake no longer builds on my Ubuntu 12.04. I
need to build binaries that are compatible with that ABI.

I see that your binary distribution of CMake 3.11 still works on
Ubuntu 12.04. Can you tell me what you do to achieve this? What are
you doing for your official builds?

Are you just using -static-libstdc++ -static-libgcc for
CMAKE_CXX_FLAGS, or is there more?

(I just noticed that ldd shows that you don't have dependencies on
libssl, libcrypto, and libz, whereas I do.)

Thanks,
Eric