Merge branch 'perl-coverage'

Merge branch 'perl-coverage'

* perl-coverage:
  Coverage for corner cases in derive_suffix.
  Coverage for user-provided _LINK variables.
  Coverage: warn about substitutions that need EXTRA_PRIMARY.
  Coverage: warn about configure substitution in EXTRA_PRIMARY.
  Coverage for syntax errors with conditionals in included fragments.
  Coverage: diagnose variables with forbidden dist_ prefix.
  Coverage for conditional `else' and `endif' arguments.