Merge branch 'master' into ng/master

Merge branch 'master' into ng/master

* master:
  maintcheck: guard against obsolete $required entries
  tests: drop requirement 'makeinfo-html'; 'makeinfo' is enough
  tests: drop requirement 'texi2dvi-o'; 'texi2dvi' is enough
  texi: require Texinfo >= 4.9, related enhancements
  docs: it's not true that DISTCHECK_CONFIGURE_FLAGS is maintainer-reserved

+ Extra non-trivial edits:

* t/silent-texi.sh ($required): Simply use 'makeinfo and 'texi2dvi',
instead of (respectively) 'makeinfo-html' and 'texi2dvi-o'.
* t/texinfo21b.sh: Fix a minor typo in heading comments.
($required): Simply use 'makeinfo instead of 'makeinfo-html'
* lib/am/texibuild.am: Merge the changes done in the master branch to
the ".texi -> .dvi" and ".texi -> .pdf" recipes into the definition
of the '$(am__texibuild_dvi_or_pdf)' make function, as well as into ...
(%.dvi: %.texi), (%.pdf: %.texi): ... calls to that function into these
pattern rules.

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