Adapted from the Ninja setup hook. This will mean that adding just to
nativeBuildInputs is enough to have a package be automatically built
with it, matching make, bmake, ninja, etc.
Currently, we have two packages in Nixpkgs that use just, dogdns and
kabeljau. kabeljau follows the expected conventions, while dogdns
does not. I expect there to be more packages using just in future,
following these conventions, because all of the packages in system76's
in-progress COSMIC desktop environment use just, and follow the
conventions.
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