Yes, Darwin (and OS X) is POSIX, see

http://www.xyroth-enterprises.co.uk/posixos.htm
SDL is used on OS X for some things, but not all.
Use #if defined(...) instead of #ifdef in most of the Selector_*.h files.

stepmania/src/arch/InputHandler/Selector_InputHandler.h

@@ -17,9 +17,9 @@
 #include "InputHandler_MonkeyKeyboard.h"
 
 // NOTE: If X11 is available, we don't use LLW_SDL, which IH_SDL depends on.
-#ifdef HAVE_X11
+#if defined(HAVE_X11)
 #include "InputHandler_X11.h"
-#elif HAVE_SDL
+#elif defined(HAVE_SDL)
 #include "InputHandler_SDL.h"
 #endif