Cargo, Windows, pkg-config and build-dependencies

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The pkg-config dependency is harmless because that small crate just calls pkg-config and parses its output. There's an issue though that its error type is String so one can't expect to discriminate failure modes. I've just been considering making a PR to add a proper error type to pkg-config.

As to pkg-config alternatives it seems to me that the only "idiomatic" alternative is building the dependency from source if pkg-config fails for whatever reason. Page Moved

The actual libgit2 build script does something slightly different and unexpected however:

    if env::var("LIBGIT2_SYS_USE_PKG_CONFIG").is_ok() {
        if pkg_config::find_library("libgit2").is_ok() {
            return
        }
    }

openssl-sys can't get away with building from source so it allows setting the search paths in env variables and calls pkg-config if none of them are set:

    let lib_dir = env::var("OPENSSL_LIB_DIR").ok();
    let include_dir = env::var("OPENSSL_INCLUDE_DIR").ok();

    if lib_dir.is_none() && include_dir.is_none() {
        // rustc doesn't seem to work with pkg-config's output in mingw64
        if !target.contains("windows") {
            if let Ok(info) = pkg_config::find_library("openssl") {
                // avoid empty include paths as they are not supported by GCC
                if info.include_paths.len() > 0 {
                    let paths = env::join_paths(info.include_paths).unwrap();
                    println!("cargo:include={}", paths.to_str().unwrap());
                }
                return;
            }
        }
        if let Some(mingw_paths) = get_mingw_in_path() {
            for path in mingw_paths {
                println!("cargo:rustc-link-search=native={}", path);
            }
        }
    }