[PATCH 1/2] m4: warn when AM_SILENT_RULES default is used

To help ease people into enabling silent rules by default, warn if
a package doesn't make an explicit selection.
---
 NEWS         | 10 ++++++++++
 m4/silent.m4 |  6 +++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/NEWS b/NEWS
index fb05ee219708..282eb9277a3d 100644
--- a/NEWS
+++ b/NEWS
@@ -3,6 +3,16 @@ please see NEWS-2.0 and start following the advice there now.
 
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+New in 1.17:
+
+* Future changes
+
+  - In preparation for silent rules being enabled by default, issue warnings
+    when a default has not been selected.  This is shown to developers (who
+    run automake for their package) and for users (who run ./configure).
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
 New in 1.16.5:
 
 * Bugs fixed
diff --git a/m4/silent.m4 b/m4/silent.m4
index 7d6036a68134..488e4fb41243 100644
--- a/m4/silent.m4
+++ b/m4/silent.m4
@@ -21,7 +21,11 @@ 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]);;
+    *) 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])])
+  ;;
 esac
 dnl
 dnl A few 'make' implementations (e.g., NonStop OS and NextStep)
-- 
2.33.0