build: support AM_SILENT_RULES

build: support AM_SILENT_RULES

* configure.ac: Add AM_SILENT_RULES.
* libltdl/configure.ac: Ditto, but only if defined.
* Makefile.am (.version, ChangeLog, README, doc/notes.txt)
(libtool, libtoolize, libtoolize.in, libltdl/Makefile.am)
(libltdl/stamp-mk, m4/ltversion.m4, tests/atconfig, tests/defs)
(tests/defs.in, tests/package.m4): Output an AM_SILENT_RULES-like
GEN when V==0.
* libltdl/Makfile.inc (libltdl/argz.h): Ditto.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>