build: silence automake build system

build: silence automake build system

* configure.ac (AM_SILENT_RULES): Invoke with "yes" as a parameter,
to activate silent rules by default.
* NEWS: Update.
* Makefile.am (sc_perl_syntax): Silence its recipe unconditionally,
similarly to what is done for other syntax checks.
(release-stats): Be silent by default *but not unconditionally*,
thanks to the use of $(AM_V_GEN).
(path-check, git-diff): Be silent by default, thanks to the use
of $(AM_V_GEN).
* m4/Makefile.am ($(top_srcdir)/m4/amversion.m4): Likewise.
* doc/Makefile.am ($(srcdir)/amhello-1.0.tar.gz): Likewise.
* tests/Makefile.am ($(srcdir)/parallel-tests.am): Likewise.
($(parallel_tests)): Likewise.