WIP: Macro to conditionally emit code depending on deployment target

[madsmtm]

Fixes upstream SSheldon/rust-objc#111.

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
}