Fix pthread_cond_destroy before macOS 10.7

macOS 10.7 returns `EINVAL` for `pthread_cond_destroy()`.

Author: catap

src/libstd/sys/unix/condvar.rs

@@ -161,14 +161,14 @@ impl Condvar {
     }
 
     #[inline]
-    #[cfg(not(target_os = "dragonfly"))]
+    #[cfg(all(not(target_os = "dragonfly"), not(feature = "macos_before_10_7")))]
     pub unsafe fn destroy(&self) {
         let r = libc::pthread_cond_destroy(self.inner.get());
         debug_assert_eq!(r, 0);
     }
 
     #[inline]
-    #[cfg(target_os = "dragonfly")]
+    #[cfg(any(target_os = "dragonfly", feature = "macos_before_10_7"))]
     pub unsafe fn destroy(&self) {
         let r = libc::pthread_cond_destroy(self.inner.get());
         // On DragonFly pthread_cond_destroy() returns EINVAL if called on