Default to 'configure --enable-silent-rules'

Default to 'configure --enable-silent-rules'

This greatly shortens the 'make' output, making it more readable
and useful.  For example, on my platform it shortens a
4125-character line "gcc -std=gnu99 -c -Demacs -I. -I. -I../lib
... emacs.c" -- a line so long that it's hard to see what's going
on or where the diagnostics are -- to just "CC emacs.o".
* INSTALL: Document this.
* configure.ac: Add AM_SILENT_RULES([yes]).
(AM_DEFAULT_VERBOSITY): Remove now-unnecessary initialization.
* etc/NEWS: Document this.
Fixes: bug#19501