shorten compiler define

Author: Free-Pascal-meets-SDL-Website

units/sdlsyswm.inc

@@ -13,7 +13,7 @@
    {$DEFINE SDL_VIDEO_DRIVER_WINDOWS}
 {$ENDIF}
 
-{$IF DEFINED (LINUX) OR DEFINED(UNIX)}
+{$IFDEF UNIX}
    {$IF NOT (DEFINED(DARWIN) OR DEFINED(ANDROID))}
       {$DEFINE SDL_VIDEO_DRIVER_X11}
    {$ENDIF}