Make zmq-sys build libzmq on Windows.

[zachlute]

This copies a pattern used by many other -sys crates which use a git submodule and build the appropriate native library.

This could probably be used on other platforms, too, but I don't have them available to test right now, and it's less of a problem on a system where you can install libzmq with a package manager anyway.

This still requires that you have CMake installed so that libzmq can use it to build.

[oblique]

I'm getting the following linkage error when compiled with --release but managed to solve it.

  = note: build_script_build-ee6089a96cb0df7d.build_script_build.2obu8exa-cgu.9.rcgu.o : error LNK2019: unresolved external symbol zmq_has referenced in function _ZN18build_script_build4main17h0d9b34f4bad19b01E

The problem is that libzmq adds /GL if build type is not Debug. So the solution is to explicitly disable that flag.

    let dst = Config::new("libzmq")
        .cxxflag("/GL-")
        .build();

    let pattern =
        match env::var("PROFILE").unwrap().as_str() {
            "debug" => format!("{}\\libzmq-v*-mt-sgd-*.lib", dst.join("lib").display()),
            "release" | "bench" => format!("{}\\libzmq-v*-mt-s-*.lib", dst.join("lib").display()),
            unknown => {
                // cmake crate defaults to release if profile is unknown
                eprintln!("Warning: unknown Rust profile={}; assuming release", unknown);
                format!("{}\\libzmq-v*-mt-s-*.lib", dst.join("lib").display())
            }
        };

[oblique]

I did the following improvements in the build script:

• Static build is disabled by default and you can enable it with static-libzmq feature in cargo.
• Fixed the compilation when release profile is used.
• Added support for other systems (tested in Linux).
• Added test case in appveyor.bat that uses static-libzmq feature.

PS: I wasn't sure how contribution goes when a PR is implemented by two people, so I opened a PR at https://github.com/zachlute/rust-zmq/pull/1 and when he merge it, my changes will appear here.

[zachlute]

Works great for me with @oblique 's changes!

[zachlute]

Closing this in favor of #241 with a bunch of improvements on my initial change.