Document some optional features in INSTALL.

Document some optional features in INSTALL.

* doc/install.texi (Basic Installation): Mention that INSTALL is
generic, and that not all packages implement all features.
Mention 'make distcheck' for maintainers, and 'make installcheck'
for users.  Mention the GNU Coding Standards.
(Installation Names): Mention DESTDIR vs. 'make prefix= install'
as ways to alter the configuration, with caveats of each.  Move
--program-prefix discussion...
(Optional Features): ...here.  Mention --enable-silent-rules and
use of make V=0.

Signed-off-by: Eric Blake <ebb9@byu.net>