> Before this change, support for silent rules was optional and activated
> only if the 'silent-rules' option was specified.
> The rationale behind that behaviour was that the silent-rules machinery
> originally only worked with make implementations supporting nested
> variables expansions, which isn't (or wasn't) a POSIX-specified features;
> so that packages wanting to be "extra-portable" couldn't use the
> silent-rules machinery at all.
> But after Paul Eggert's commit 'v1.11-598-g8493499' of 25-12-2011,
> "silent-rules: fallback for makes without nested vars" (motivated
> by automake bugs #9928 and #10237), Makefiles generated when the
> 'silent-rules' option is enabled can work also with those make
> implementations that don't support nested variables expansion (albeit
> the configure-time selected verbosity cannot be overridden at make
> runtime in such case).
> In light of that, and since silent rules are disabled by default even
> when the 'silent-rules' option is given (unless the developer explicitly
> makes them enabled by default be calling "AM_SILENT_RULES([yes])" in
> configure.ac), we can now have the support for silent rules always
> The 'silent-rules' option will thus become a no-op, but will reaming a
> valid option for the time being, for better backward-compatibility.
> A collateral effect of this change is that the use of the 'silent-rules'
> option will not anymore automatically disable the warnings in the
> 'portability-recursive' category.
> The present change is basically a backport of the Automake-NG commit
> 'v1.11d-75-g61ca923' of 19-04-2012, "[ng] silent rules: support for
> them is always active now". Stemmed by this thread on the Automake-NG
> * automake.in: Enable silent-rules machinery unconditionally. Reword
> some comments. Do not switch off warnings in the category
> 'portability-recursive' anymore if the 'silent-rules' option is
> * m4/init.m4: Call 'AC_REQUIRE' on 'AM_SILENT_RULES' unconditionally.
> * lib/Automake/Options.pm: Recognize 'silent-rules' as a dummy option.
> Do not bother anymore checking that it is set in 'configure.ac'.
> * t/silent.sh: Remove now-redundant calls to AM_SILENT_RULES.
> * t/silent2.sh: Likewise.
> * t/silent3.sh: Likewise.
> * t/silent4.sh: Likewise.
> * t/silent8.sh: Likewise.
> * t/silent9.sh: Likewise.
> * t/silent-lex.sh: Likewise.
> * t/silent-many-gcc.sh: Likewise.
> * t/silent-many-generic.sh: Likewise.
> * t/silent-yacc-headers.sh: Likewise.
> * t/silent-yacc.sh: Likewise.
> * t/silentcxx-gcc.sh: Likewise.
> * t/silentcxx.sh: Likewise.
> * t/silentf77.sh: Likewise.
> * t/silentf90.sh: Likewise.
> * t/help-silent.sh: Likewise.
> * t/silent-configsite.sh: Likewise, and other minor adjustments.
> * t/silent7.sh: Likewise. Also, extend to check that silent rules
> are disabled by default, and remove obsoleted checks about the
> 'silent-rules' option being rejected in 'AUTOMAKE_OPTIONS'.
> * t/silent6.sh: Remove now-redundant calls to 'AM_SILENT_RULES'.
> Call automake with '-Wno-portability-recursive'. Remove other
> obsoleted checks.
> * t/dollarvar.sh: Don't expect the use of 'AM_SILENT_RULES' in
> 'configure.ac' to disable warning in the 'portability-recursive'
> category anymore.
> * t/compile_f_c_cxx.sh: Adjust to avoid a spurious failure.
> * t/silent-amopts.sh: Remove as obsolete.
> * t/silent-nowarn.sh: Likewise.
> * t/list-of-tests.mk: Adjust.
> * NEWS: Update.
> * doc/automake.texi: Update and simplify accordingly.
> * configure.ac (AM_INIT_AUTOMAKE): Remove 'silent-rules' option.