Patch from Sylvain to fix clang warnings

src/thread/SDL_systhread.h

@@ -55,7 +55,7 @@ extern void SDL_SYS_WaitThread(SDL_Thread * thread);
 extern void SDL_SYS_DetachThread(SDL_Thread * thread);
 
 /* Get the thread local storage for this thread */
-extern SDL_TLSData *SDL_SYS_GetTLSData();
+extern SDL_TLSData *SDL_SYS_GetTLSData(void);
 
 /* Set the thread local storage for this thread */
 extern int SDL_SYS_SetTLSData(SDL_TLSData *data);

src/thread/SDL_thread_c.h

@@ -82,7 +82,7 @@ typedef struct {
    This is only intended as a fallback if getting real thread-local
    storage fails or isn't supported on this platform.
  */
-extern SDL_TLSData *SDL_Generic_GetTLSData();
+extern SDL_TLSData *SDL_Generic_GetTLSData(void);
 
 /* Set cross-platform, slow, thread local storage for this thread.
    This is only intended as a fallback if getting real thread-local

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>