Check for O_CLOEXEC definition in fcntl.h at configuration time,

If not available, define it internally as 0 (in SDL_internal.h).
Define it as available for macOS >= 10.7 in SDL_config_macosx.h.

CMakeLists.txt

@@ -993,6 +993,10 @@ if(UNIX OR APPLE)
   endif()
 endif()
 
+if(UNIX OR APPLE OR HAIKU OR RISCOS)
+  CheckO_CLOEXEC()
+endif()
+
 if(SDL_JOYSTICK)
   if(SDL_VIRTUAL_JOYSTICK)
     set(HAVE_VIRTUAL_JOYSTICK TRUE)