Build system, autotest.am: Support Automake 1.11 `silent-rules'.

  * Introduce a new macro mdw_SILENT_RULES which enables the use of
    the `silent-rules' feature if it's available.  If the feature is
    /not/ available, then make sure that AM_DEFAULT_VERBOSITY is
    defined, in order to avoid errors about recursive variable-expansion
    in custom silent-rules actions.

  * Use this machinery to implement `silent-rules' building in the
    package itself.