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.

include/SDL_config.h.in

@@ -215,6 +215,7 @@
 #define HAVE_STDINT_H 1
 #endif /* HAVE_LIBC */
 
+#undef HAVE_O_CLOEXEC
 #undef HAVE_ALTIVEC_H
 #undef HAVE_DBUS_DBUS_H
 #undef HAVE_FCITX
@@ -234,14 +235,13 @@
 #undef HAVE_DXGI_H
 #undef HAVE_WINDOWS_GAMING_INPUT_H
 #undef HAVE_XINPUT_H
+#undef HAVE_XINPUT_GAMEPAD_EX
+#undef HAVE_XINPUT_STATE_EX
 
 #undef HAVE_MMDEVICEAPI_H
 #undef HAVE_AUDIOCLIENT_H
 #undef HAVE_SENSORSAPI_H
 
-#undef HAVE_XINPUT_GAMEPAD_EX
-#undef HAVE_XINPUT_STATE_EX
-
 /* SDL internal assertion support */
 #undef SDL_DEFAULT_ASSERT_LEVEL