Shorten and improve INSTALL

Shorten and improve INSTALL

Rewrite install.texi so that INSTALL is shorter and hopefully
more useful for new installers.
* doc/install.texi: When not building the Autoconf manual, put
copyright notice at the end, and omit external references as they
do not work in INSTALL.
Avoid long URLs as they do not work well in INSTALL or
in info files.  Be more consistent (though not entirely
consistent) about “system” vs “architecture”.
(System Types, Building for a different system type)
(Troubleshooting the Build Type): Coalesce into a single section
and rewrite for clarity and brevity.  Mention build-aux.  Do not
mention --target here as it’s too rare to make the cut.