Update and polish the documentation.

Update and polish the documentation.

* doc/autoconf.texi: Use @sc where upper case names were used.
Remove a few useless `dnl'.
Convert some `dnl' into comments.
Promote `$(foo)' in Makefiles, not `${foo}'.
Promote `$foo' in shell scripts, not `${foo}'.
Promote `foo = bar' in Makefiles, not `foo=bar'.
(Language Choice): Document `AC_LANG', move `AC_LANG_C',
`AC_LANG_CPLUSPLUS' and `AC_LANG_FORTRAN77' to
(Old Macros): here.
* doc/install.texi: Fix a typo in the specification of the system
quadruples.