If not, WDYM exactly by "check whether cl-lib is available"?
Passing a non-nil NOERROR argument to a top-level

  (eval-when-compile (require 'cl-lib nil t))

still causes 'make bootstrap' to fail, and writing

  (when (require/featurep/fboundp ...)
    (cl-assert ...))

in custom-available-themes doesn't make sense to me, especially given
that cl-assert is autoloaded.

Am I misunderstanding something?  Sorry about my slow uptake on this;
I'm not deliberately trying to be obtuse, rather trying to learn what
the best approach is here and why.

Thanks,

-- 
Basil