Fix --libdir issues

[brson]

The way rust and rust-installer handles --libdir is broken, and at least in Gentoo's case is unusable without patching.

See Gentoo's rust-installer patch. And an old rust-installer PR.

re https://internals.rust-lang.org/t/perfecting-rust-packaging-the-plan/2767

[wthrowe]

It appears that --libdir fails on recent nightlys even with Gentoo's patch: gentoo/gentoo-rust#29