Disable Make jobserver support on OSX

On OSX, CMake (via libuv) spawns child processes, including make, via
the Apple-specific `posix_spawn` attribute
`POSIX_SPAWN_CLOEXEC_DEFAULT`. This blocks make from accessing all
non-stdio file descriptors from the parent process, including those of
the jobserver.

Because make was getting passed jobserver information via MAKEFLAGS but
was unable to access the jobserver, it prints an error like the
following and proceeds to run a single-threaded build:

```
make: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.
```

As a workaround, disable jobserver support on OSX so that cmake-rs
instead passes `--parallel $NUM_JOBS` to CMake, allowing for a faster
multi-threaded build.

Author: kesyog

src/lib.rs

@@ -837,16 +837,18 @@ impl Config {
         let mut use_jobserver = false;
         if fs::metadata(build.join("Makefile")).is_ok() {
             match env::var_os("CARGO_MAKEFLAGS") {
-                // Only do this on non-windows and non-bsd
+                // Only do this on non-windows, non-bsd, and non-osx
                 // On Windows, we could be invoking make instead of
                 // mingw32-make which doesn't work with our jobserver
                 // bsdmake also does not work with our job server
+                // On OSX, CMake blocks propagation of the jobserver's file descriptors to make
                 Some(ref makeflags)
                     if !(cfg!(windows)
                         || cfg!(target_os = "openbsd")
                         || cfg!(target_os = "netbsd")
                         || cfg!(target_os = "freebsd")
-                        || cfg!(target_os = "dragonfly")) =>
+                        || cfg!(target_os = "dragonfly")
+                        || cfg!(target_os = "macos")) =>
                 {
                     use_jobserver = true;
                     cmd.env("MAKEFLAGS", makeflags);