cmake: fix uses of undefined macro identifiers (-Wundef)

test/checkkeys.c

@@ -284,7 +284,7 @@ int main(int argc, char *argv[])
 
     textwin = SDLTest_TextWindowCreate(0, 0, 640, 480);
 
-#if __IPHONEOS__
+#ifdef __IPHONEOS__
     /* Creating the context creates the view, which we need to show keyboard */
     SDL_GL_CreateContext(window);
 #endif