Patch from Sylvain to fix clang warnings

src/thread/pthread/SDL_syscond.c

@@ -129,7 +129,7 @@ SDL_CondWaitTimeout(SDL_cond * cond, SDL_mutex * mutex, Uint32 ms)
     switch (retval) {
     case EINTR:
         goto tryagain;
-        break;
+        /* break; -Wunreachable-code-break */
     case ETIMEDOUT:
         retval = SDL_MUTEX_TIMEDOUT;
         break;

src/thread/pthread/SDL_systls.c

@@ -20,6 +20,7 @@
 */
 #include "../../SDL_internal.h"
 #include "SDL_thread.h"
+#include "../SDL_systhread.h"
 #include "../SDL_thread_c.h"
 
 #include <pthread.h>