Merge branch 'master' into ng/master

Merge branch 'master' into ng/master

* master:
  docs: fix warnings given by Texinfo 5.x
  fixlet: remove leftover comment fragment
  master branch: we are going to become Automake 2.0
  maint branch: we are going to become Automake 1.14
  maint: version bump after beta release 1.13.1d
  release: beta release 1.13.1d (will become 1.13.2)
  NEWS: document more robust handling/recognition of make options
  tests: typofixes in comments in t/preproc-c-compile.sh
  tests: remove bashism from a test
  maint: targets and recipes to simplify testing on real-world packages
  build: preparatory refactoring
  build: tiny reduction in code duplication
  make flags analysis: handle more options with args
  make flags analysis: use simpler variable names
  make flags analysis: whitespace changes
  make flags analysis: embed in a subshell
  make flags analysis: be more robust
  make flags analysis: cater to GNU make 3.83 (still unreleased as of now)
  tests: expose weaknesses in make flags analysis
  tests: improve debugging output in checks on make flags analysis
  make flags analysis: refactor, to reduce code duplication
  tests: avoid one tricky use of "make -e"
  cosmetics: remove few trailing whitespace occurrences
  typofix: in NEWS (extra whitespace)
  compat: substitute '@mkdir_p@' as '$(MKDIR_P)' unconditionally
  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)

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>