PHP 8.0 compatibility. Reference to undeclared constant \THEMEKEY_PAGECACHE_SUPPORTED

This is a bit strange, I need more info why this happens.

The function crumbs_themekey_properties() should only be called if themekey is enabled.
And THEMEKEY_PAGECACHE_SUPPORTED is defined in themekey.module.
The constant is globally defined by themekey since 7.x-1.5, commit c9e55075e3bb6 happened in 2011.

Are you using an older version of themekey?
Or is themekey not enabled, and the hook is called nevertheless?

Also:
- Doesn't seem to be related to PHP 8.0 compatibility.
- Your patch has wrong indentation.

Undeclared constant will cause fatal error in php 8.0+. So i added additional checking that this constant defined, in case if it will be deleted or renamed.

Then we would also need to call function_exists() whenever we want to call a function from another module.
Should we do that, or should we rely on new versions respecting backwards compatibility? I think at this point in the Drupal 7 life cycle we don't need to worry that the constant might be removed in themekey :)

If crumbs_themekey_properties() is a hook invoked only from the ThemeKey module, which defines that constant in the themekey.module file, that error message could only happen in a case: when hook_themekey_properties() is invoked but the themekey.module file isn't loaded. Could this happen?