Merge branch 'master' into next

Merge branch 'master' into next

* master:
  Rewrite maintainer-check in separate tests, parallelizable.
  Fix maintainer-check failure.
  Fix latex warning in manual.
  Sync auxiliary files from upstream.
  manual: minor cleanups.
  Sane (un)install for empty directory variables.
  Ensure that empty directory variables work with empty content variables.
  Use --tag=FC with libtool also for .f90 files.
  DISTFILES containing a directory and files in that directory.
  Fix a documentation typo.
  Define AM_RECURSIVE_TARGETS, for gnulib's GNUmakefile.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>