Merge branch 'je-silent' into next

Merge branch 'je-silent' into next

* je-silent:
  Rename `silent' mode to `silent-rules' mode.
  `silent' mode unconditionally overrides portability-recursive.
  Drop extra line from compile rules when `silent' is not used.
  Keep `--silent-rules' across triggered automake reruns.
  Provide variables for silencing of user rules.
  Redo variable naming for `silent' machinery.
  Let -Wportability turn on/off the portability-recursive channel.
  New automake command line option `--silent-rules'.
  Documentation and tests for the `silent' option.
  Implement `silent' build rules.
  New channel `portability-recursive'.