bug#70484: Fix compatibility of build-aux/make-info-dir script

[Felix Janda]

Hi,

I'm reporting the bug in the build scripts:

https://bugs.gentoo.org/930086

To summarize, the build-aux/make-info-dir script uses the substitution
${@?}, which is incompatible with mksh (and possibly other shells) and
undefined behavior in the proposed next POSIX standard.

A possible fix is to check $# at the beginning of the script, and then
simplify the rest of the script.

Felix