Merge branch 'master' into ng/master

Merge branch 'master' into ng/master

* master:
  dist: fix bug#18286 "distcheck fails to detect missing files"
  tests: expose automake bug#18286 "distcheck fails to detect missing files"
  include: fix bug in handling of user-defined makefile fragments generation
  tests: expose bug in handling of user-defined makefile fragments generation

+ Extra non-trivial changes:

* lib/am/distcheck.mk: Backport the changes done to lib/am/distdir.am
in commit v1.14.1-4-g01a7a4a (fix for automake bug#18286 "distcheck
fails to detect missing files").

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