cross-compilation broken with latest release

[trinity-1686a]

Cross compilation is somewhat broken in 0.10.0 due to the use of a build-script to set cfg flags.

To reproduce, on linux (no need for a full CC toolchain with clang, rustc is enough):

rustup target add x86_64-apple-darwin
cargo check --target x86_64-apple-darwin

A build-script was introduced, I assume to reduce lengthy cfg!(). Sadly it doesn't work as intended. target_os, target_env &co inside a build-script correspond to the host, not the real target, as the build-script is always compiled to be run on the host. This leads to rlimit__has_prlimit64 being set, based on the linux host having that feature, when the target is actually macOS, which shouldn't have it (it seems)

[trinity-1686a]

thanks for the quick fix! ❤️