Fixed a whole slew of compiler warnings that -Wall exposed.

src/thread/pthread/SDL_systhread.c

@@ -18,6 +18,7 @@
      misrepresented as being the original software.
   3. This notice may not be removed or altered from any source distribution.
 */
+
 #include "SDL_config.h"
 
 #include <pthread.h>
@@ -31,6 +32,8 @@
 #include <sys/time.h>
 #include <sys/resource.h>
 #include <sys/syscall.h>
+#include <unistd.h>
+extern int pthread_setname_np (pthread_t __target_thread, __const char *__name) __THROW __nonnull ((2));
 #endif
 
 #include "SDL_platform.h"