AC_INIT behaves confusingly with an empty version argument

[Tim Cuthbertson]

The fish-shell[0] configure.ac file contains the following call:

AC_INIT(fish,
        m4_esyscmd([cut -f 3 -d ' ' FISH-BUILD-VERSION-FILE | tr -d '\n']),
        address@hidden)

Under certain circumstances (an unrelated bug), the second (version)
argument can end up being the empty string.

This is obviously not ideal use, but autoconf lets it go way too far.

Seeing that the second arg is empty, it treats the call as if it were
just AC_INIT(fish), an undocumented variant of the function which was
apparently deprecated over a decade ago[1]. This produces a
dramatically different ./configure script which fails because it can't
find a file named "fish" (as $ac_unique_file), which doesn't resemble
the real problem at all.

Rather than simply testing for an empty second argument, it would be
more correct to check the number of arguments- i.e if $# equals 1, act
like the old API. Otherwise, autoconf should act like the new API
(with an error reported for an empty version string, if that's a
required argument).

Cheers,
 - Tim.

[0]: https://github.com/fish-shell/fish-shell
[1]: http://stackoverflow.com/a/17664203/144135