c11: Fix missing pthread_mutex_timedlock declaration warnings on MacOSX.

include/c11/threads_posix.h

@@ -41,7 +41,7 @@ Configuration macro:
     Use pthread_mutex_timedlock() for `mtx_timedlock()'
     Otherwise use mtx_trylock() + *busy loop* emulation.
 */
-#if !defined(__CYGWIN__)
+#if !defined(__CYGWIN__) && !defined(__APPLE__)
 #define EMULATED_THREADS_USE_NATIVE_TIMEDLOCK
 #endif
 
@@ -196,6 +196,12 @@ mtx_lock(mtx_t *mtx)
     return thrd_success;
 }
 
+static inline int
+mtx_trylock(mtx_t *mtx);
+
+static inline void
+thrd_yield(void);
+
 // 7.25.4.4
 static inline int
 mtx_timedlock(mtx_t *mtx, const xtime *xt)