[whateveracct]

You write some Nix code to download said tarball and build it as you said. In fact, the ever present `mkDerivation` often Just Works for autotools and cmake projects.

[pxc]

You can also specify paths to local sources, of course! Instead of src = fetchgit... or whatever, you can just write

  src = ./some-source-dir;

or

  src = ./.;

or whatever