- Perl is used at build time, so must be in `nativeBuildInputs`. It's
not used at run time so it should not be in `buildInputs`, too.
- Don't treat headers like a compiler---use the build and host
platforms not host and target. Perhaps that would make sense if every
library's headers could be a separate derivation, but since that is
not feasible, best to keep the implementation and interface in the
same stage.
To do this, we used `stdenvNoCC` to get rid of the normal toolchain,
and added a dependency for the toolchain targeting the build platform
--- `buildPackages.stdenv.cc` --- thus everything is effectively slid
a stage black.
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