ac 2.57: documentation build dependencies

[Tim Pierce]

Hi folks --

There is a non-optimal dependency chain for autoconf.info in the
autoconf-2.57 distribution:

autoconf.info   depends on      version.texi
version.texi    depends on      stamp-vti
stamp-vti       depends on      ../configure

Since the top-level configure script is newer than stamp-vti, make
ends up wanting to rebuild autoconf.info out of the box.  I'm not sure
what the best solution would be, but if when the distribution is built
the top-level configure script could be generated before descending
the subdirectories, it would help.

Also: if make attempts to build autoconf.info with makeinfo 4.0 or
older, makeinfo will fail as follows:

autoconf.texi:70: Unknown command `copying'.
autoconf.texi:94: Unmatched address@hidden'.
autoconf.texi:145: Unknown command `insertcopying'.
autoconf.texi:3802: Unknown command `verbatim'.
autoconf.texi:3817: Misplaced {.
autoconf.texi:3821: Misplaced }.
autoconf.texi:3823: Bad argument to `end', `verbatim', using `defmac'.
autoconf.texi:3823: Unmatched address@hidden'.
autoconf.texi:4281: Unknown command `verbatim'.
autoconf.texi:4298: Bad argument to `end', `verbatim', using `defmac'.
autoconf.texi:4298: Unmatched address@hidden'.
autoconf.texi:4519: Unknown command `verbatim'.
autoconf.texi:4526: Unmatched address@hidden'.
autoconf.texi:7739: Unknown command `verbatim'.
autoconf.texi:7747: Unmatched address@hidden'.
makeinfo: Removing output file `autoconf.info' due to errors; use --force to 
preserve.

As a result there is no autoconf.info file left at all.  I suggest
adding --force to AM_MAKEINFOFLAGS.

It may also be helpful to check for makeinfo 4.2 or later in
configure.ac, maybe something like this:

saveIFS=$IFS; IFS="."
set `makeinfo --version | awk 'NR == 1 { print $NF }'`
majver=$1
minver=$2
IFS=$saveIFS

if test $majver -lt 4 || test $majver -eq 4 && test $minver -lt 2
then
        AC_MSG_WARN([need texinfo 4.2 or later to rebuild autoconf.info])
fi

I hope these reports are helpful.  Thanks much for all your hard work
on autoconf!

-- twp