configure: if no perl, disable unity and shell completion, related tidy ups

configure: if no perl, disable unity and shell completion, related tidy ups

Also:
- GHA/linux, macos: test `install` with autotools too.
- GHA/linux, macos: enable fish and zsh completion in an autotools job.
- scripts: drop dynamic perl checks redundant after this patch.
- scripts: drop two interim variables.
- autotools: make `build-certs` target check perl first.
- autotools: replace `$(PERL)` with `@PERL@` to match the pattern used
  in most automake scripts. For consistency. This makes `PERL` be
  defined at configure-time, as opposed to make-time, for these cases
  now.

Closes #18141