Use libc++ on Mac OS X

Could we change to using libc++ on Mac OS X. The current crate does not work on Catalina.

I think we just need to add another option here:

Line 40 in a358c33

println!("cargo:rustc-link-lib=dylib=stdc++");

I guess this would change to

 println!("cargo:rustc-link-lib=dylib=c++");

on Mac OS X.

Just to confirm I have the same issue on Catalina, but I'm able to build by applying the suggested change. The cc crate has a piece of logic that selects the right stdlib, ie. libc++ on darwin/bsd, libstdc++ elsewhere: https://github.com/alexcrichton/cc-rs/blob/master/src/lib.rs#L2248-L2259. I think the same rules should be used here.

fixed #18 .

Yet, the crate failed to compile on MinGW.
And BSD environment is not tested yet.

freebsd works well

I will close this then and open a new issue for mingw