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/timer/unix/SDL_systimer.c

@@ -158,9 +158,9 @@ SDL_GetPerformanceFrequency(void)
         freq /= mach_base_info.numer;
         return freq;
 #endif
-    } else {
-        return 1000000;
-    }
+    } 
+        
+    return 1000000;
 }
 
 void