Tested building pkgsCross.x86_64-netbsd.netbsd.libm from x86_64-linux.
At some point we should probably set SHLIBINSTALLDIR in the setupHook,
but I think I'd like to get everything working first, and then make
changes that affect all the builds like that. It's easier to spot
regressions when you know _everything_ worked before.
5c24380395