WIP: Macro to conditionally emit code depending on deployment target

[madsmtm]

Fixes #266.

Useful for various optimizations and feature-gating functionality depending on OS version.

The expansion is roughly:

#[cfg_available(macOS(10.9), ...)] fn my_fn() {} // MACOSX_DEPLOYMENT_TARGET=10.9 #[cfg(any(target_os = "macos"))] fn my_fn() {} // MACOSX_DEPLOYMENT_TARGET=10.7 #[cfg(any(__not_available))] fn my_fn() {} // MACOSX_DEPLOYMENT_TARGET not set // Available on Aarch64, since that has a higher default deployment target #[cfg(any(not(all(target_os = "macos", not(target_arch = "aarch64")))))] fn my_fn() {}

Further improvement would be an available! macro that checks the runtime version if the deployment target is not high enough, like this:

if available!(macOS(10.9)) { // Do something } else { // Fallback } // MACOSX_DEPLOYMENT_TARGET=10.9 if cfg!(any(target_os = "macos")) { // Do something } else { // Fallback } // MACOSX_DEPLOYMENT_TARGET=10.7 if runtime::check_version(10, 9, 0) { // Do something } else { // Fallback }