Use feature guard instead of environment variable to enable static link

[BusyJay]

Using environment variable will have following shortcomings:

• A rebuild won't be triggered when set an environment variable;
• If some project always prefer linking statically, there is no clean way to build such a binary.

Features guard can solve both perfectly, and also a rusty way.

[sfackler]

This change would probably need to be made in pkg-config first.

[BusyJay]

I don't get it. OPENSSL_STATIC is not used by pkg-config, right? I think it should be simply adding if cfg!(feature = "static-link") at line https://github.com/sfackler/rust-openssl/blob/master/openssl-sys/build.rs#L395.

[sfackler]

OPENSSL_STATIC is used by pkg-config: https://docs.rs/pkg-config/0.3.9/pkg_config/.

[BusyJay]

Oh, I see. Sorry for my carelessness. Since pkg_confg supports passing --static explicitly, it should be able to check the features and add the flags without breaking old behavior.

[alexcrichton]

I'd personally find build configuration like this to be more naturally done as environment variables than as features. For example someone building a crate shouldn't need to modify Cargo.toml to configure the build. Additionally crates themselves shoudn't really care whether OpenSSL is linked statically or dynamically, that's more of a question of whomever is generating the final build artifact.

I think the biggest problem with using environment variables, build scripts don't get rerun if they're changed, has now been fixed