bug in protobuf feature build for Windows

[williballenthin]

#14 introduces support for building the protobuf code on Windows, but I think its subtly broken: using target_os in build.rs reflects the OS of the host building the build script, not the final program. ref

So, this code should be updated: https://github.com/CerebusOSS/ella/blob/0343b4f54f1f81d093a27da79192ed9c468fffec/ella-server/build.rs#L2C1-L5C66

perhaps to if std::env::var_os(key: "CARGO_CFG_WINDOWS").is_some() {...} else {...}

I encountered this while trying to get my project to build prost/protobuf-src for Windows, and your work got me nearly the whole way there. Thank you!