Makes sense to have -ffunction-sections and co. -Wl,--gc-sections and -Wl,--icf=safe can be omitted though as those only apply to linking by the cc crate, but we only build a staticlib here. They don't have any effect on linking done by rustc.

This should probably forward the OPT_LEVEL env var instead of adding a feature.

Didn't notice this commit. Maybe squash it into the first commit?

I've asked at https://rust-lang.zulipchat.com/#narrow/channel/246057-t-cargo/topic/read.20lto.20config.20from.20build.20script if there is a way to read the LTO state from cargo without having to use feature flags. In any case this won't actually LTO between bzip2 and any rust code. That is only possible when using the rust port of bzip2 rather than the C original or by using -Clinker-plugin-lto and making sure that the exact right clang version is used to build the C code.

Turns out the cc crate itself nowadays handles -Clto and -Copt-level already, so the second and third commit can be dropped. Only the first and fourth commit (which should be squashed) are still necessary.