enable silent build output by default

Enable the automake feature to produce silent output by default.
When compiling code, we now see things like:
$ make
  CC       addgrps.o
  CC       age.o
  CC       audit_help.o
...

This can be disabled via configure's --disable-silent-rules or
by passing V=1 to make.

Custom output (like in the man subdirs) don't (yet) respect this
feature.  More work will be needed to clean those up.

configure.ac

@@ -3,6 +3,7 @@ AC_PREREQ([2.64])
 AC_INIT([shadow], [4.4], [pkg-shadow-devel@lists.alioth.debian.org], [],
 	[https://github.com/shadow-maint/shadow])
 AM_INIT_AUTOMAKE([1.11 foreign dist-xz])
+AM_SILENT_RULES([yes])
 AC_CONFIG_HEADERS([config.h])
 
 dnl Some hacks...