Merge branch 'fix-pr12554' into branch-1.13.2

Merge branch 'fix-pr12554' into branch-1.13.2

* fix-pr12554:
  tests: avoid a spurious error with Solaris make
  subdirs: don't return false positives for the '-k' option's presence
  header-vars: recognize more make flags ('-k' in particular)
  header-vars: simplify how make flags are determined
  tests: remove dead code from t/make-dryrun.tap
  header-vars: new variable $(am__running_with_option)
  tests: expose bug#12554 (false positives for presence of '-k' make option)