Ensure cross-build compatibility of build.rs

[Guiguiprim]

This changes make it possible to cross compile for MacOS.

[hoodie]

thanks. what exactly is the difference?

[Guiguiprim]

When cross-compiling on platform A for platform B, the code itself is build for B but build.rs are build for A as they need to be executed during the compilation process. As a result inside build.rs, cfg!(target_os) will have a value related to A whereas std::env::var("CARGO_CFG_TARGET_OS") will have a value related to B.

In the build.rs we want to do stuff if we are building for macos not from macos, and the right way for testing it is through the adequate env and not cfg, this sadly is a error commonly done.