Replace build.rs with link attributes

[newpavlov]

Closes #204.

iOS code already had link attributes. To be extra safe I would prefer for someone to verify that this change does not break iOS targets.

[Absolucy]

I'm having issues related to note: ld: framework not found Security with iOS targets + LTO, so this seems like it'd actually fix things.

Can't test as my dep tree pulls in 0.1 and 0.2 of getrandom, though, so [patches.crates-io] doesn't work

[josephlr]

This looks correct. After reading the documentation for the cargo:rustc-link-lib=[KIND=]NAME instruction in build scripts, the #[link] attribute, and the -l rustc flag, I can confidently say that:

• Adding "cargo:rustc-link-lib=framework=Security" to build.rs
• Adding #[link(name = "Security", kind = "framework")] to an extern "C" block

do the exact same thing, namely passing -l framework=Security to rustc.

[josephlr]

Can't test as my dep tree pulls in 0.1 and 0.2 of getrandom, though, so [patches.crates-io] doesn't work

Does this mean you would also like to see this change backported to 0.1?

[Lokathor]

if 0.1 is getting new releases there's no reason to not also do this in 0.1