autogoo: replace AC_HELP_STRING with AS_HELP_STRING

Niels De Graefrequested to merge
nielsdg/obsolete-autoconf-shit into master

Autoconf made the AC_HELP_STRING macro obsolete many moons ago in Autoconf. It should be replaced with AS_HELP_STRING, so let's do that.

This fixes quite a few warnings in the configure output

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