[PATCH 2/2] m4: enable silent build rules by default

This has been available since automake 1.11 released over a decade
ago.  Let's flip the default to enable silent builds by default.

NB: The "1.20" version is a placeholder.  With the warning for devs
in 1.17, we can wait a while before rolling this out.
---
 NEWS         | 9 +++++++++
 m4/silent.m4 | 6 +-----
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/NEWS b/NEWS
index 282eb9277a3d..c7e7ba3ed52c 100644
--- a/NEWS
+++ b/NEWS
@@ -3,6 +3,15 @@ please see NEWS-2.0 and start following the advice there now.
 
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+New in 1.20:
+
+* Miscellaneous changes
+
+  - Silent rules are enabled by default.  They can be disabled with the call
+    `AM_SILENT_RULES([no])`.
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
 New in 1.17:
 
 * Future changes
diff --git a/m4/silent.m4 b/m4/silent.m4
index 488e4fb41243..419d3ebc6181 100644
--- a/m4/silent.m4
+++ b/m4/silent.m4
@@ -21,11 +21,7 @@ AS_HELP_STRING(
 case $enable_silent_rules in @%:@ (((
   yes) AM_DEFAULT_VERBOSITY=0;;
    no) AM_DEFAULT_VERBOSITY=1;;
-    *) AM_DEFAULT_VERBOSITY=m4_if([$1], [yes], [0], [1])
-  m4_ifblank([$1], [dnl
-   m4_warn([gnu], [Future automake versions will enable silent rules by default; select a default with AM_SILENT_RULES])
-   AC_MSG_WARN([Future versions will enable silent rules by default; use --disable-silent-rules to keep verbose output])])
-  ;;
+    *) AM_DEFAULT_VERBOSITY=m4_if([$1], [no], [0], [1]);;
 esac
 dnl
 dnl A few 'make' implementations (e.g., NonStop OS and NextStep)
-- 
2.33.0