Add `configure --config-cache', `-C' as a shortcut for

Add `configure --config-cache', `-C' as a shortcut for
`--cache-file=config.cache'.

* acgeneral.m4 (_AC_INIT_PARSE_ARGS): Let `configure' support
`--config-cache' and `-C'.
Report them in `--help'.
Fix the display of missing argument.
(_AC_OUTPUT_SUBDIRS): Adjust.
* doc/install.texi (Invoking configure): Rename the node as
(Running configure scripts): this, the name of the chapter.
(Operation Controls): Rename as...
(Invoking configure): this.
Document the new options.
* doc/autoconf.texi (direntry): Huh?  What is this `aclocal' doing
here?  Point to `Invoking configure'.
(Cache Files): No need to document the disabling of the cache.