Calls to setpriority and getpriority have incorrect types on macOS

[lovesegfault]

When trying to compile on the latest nightly (or stable) on macOS with the latest libc I get the following errors

   Compiling scheduler v0.1.3 (file:///Users/bemeurer/src/rust-scheduler)
error[E0308]: mismatched types
  --> src/resource.rs:30:32
   |
30 |     match unsafe { setpriority(c_which as u32, who as id_t, priority) } {
   |                                ^^^^^^^^^^^^^^ expected i32, found u32
error[E0308]: mismatched types
  --> src/resource.rs:50:41
   |
50 |     let priority = unsafe { getpriority(c_which as u32, who as id_t) };
   |                                         ^^^^^^^^^^^^^^ expected i32, found u32
error: aborting due to 2 previous errors

Looking at the libc docs for both setpriority() and getpriority() we find that they both take an __priority_which_t, which is an alias to c_uint, which in turn is an alias to u32. All this indicates that the current calls are correct, and in that line the project compiles fine on Linux; just not on macOS, which seemingly expects an i32 there.

The only fix I can think of is conditional compilation through #[cfg].