Perl has an env hook[1]. Passing the paths manually without putting them
in buildInputs is harder to understand, plays less nicely with dev
shells, and is less build-generic.
Produced identical output on my x86_64-linux machine, and on my
aarch64-darwin machine was identical save for the derivation output path
which gets embedded into the .dylib Mach-O.
Change-Id: Ib313caa5a6f0b0e3154ce6f05379033920d0d290
