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

[vszakats]

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.