rwlock: Added SDL_rwlock API for shared locks.

src/thread/pthread/SDL_sysmutex_c.h

@@ -23,9 +23,18 @@
 #ifndef SDL_mutex_c_h_
 #define SDL_mutex_c_h_
 
+#if !(defined(SDL_THREAD_PTHREAD_RECURSIVE_MUTEX) || \
+    defined(SDL_THREAD_PTHREAD_RECURSIVE_MUTEX_NP))
+#define FAKE_RECURSIVE_MUTEX
+#endif
+
 struct SDL_mutex
 {
     pthread_mutex_t id;
+#ifdef FAKE_RECURSIVE_MUTEX
+    int recursive;
+    pthread_t owner;
+#endif
 };
 
 #endif /* SDL_mutex_c_h_ */