[Super Cache] Show missing modules notice when using header related constants

### Impacted plugin

Super Cache

### What

When using WPSC_CACHE_CONTROL_HEADER to set a custom header, if the related apache modules are not enabled, the header won't be set. We could show a notice that related apache modules are missing.

### How

There's already a missing modules notice shown, but that's only visible when enabling Advanced caching:

Since the constant is present on each WP load, it should be fairly easy to update the check that handles showing the notice, so it always shows if something's misconfigured.