Fixes bug #2074 - Thanks Sylvain!

SDL_syssem.c:159 comparison of unsigned expression >= 0 is always true
Solved by comparing unsigneds directly

SDL_systimer.c:164: warning: control may reach end of Compile
Solved by returning the default value if all else fails.

SDL_androidgl.c:41:1: warning: type specifier missing, defaults to 'int'
SDL_androidgl.c:47:1: warning: control reaches end of non-void function
Solved by adding void return type to the function implementation

src/thread/pthread/SDL_syssem.c

@@ -156,7 +156,7 @@ SDL_SemWaitTimeout(SDL_sem * sem, Uint32 timeout)
 #else
     end = SDL_GetTicks() + timeout;
     while ((retval = SDL_SemTryWait(sem)) == SDL_MUTEX_TIMEDOUT) {
-        if ((SDL_GetTicks() - end) >= 0) {
+        if (SDL_GetTicks() >= end) {
             break;
         }
         SDL_Delay(0);