AC_HELP_STRING -> AS_HELP_STRING

AS_HELP_STRING seems to have been in autoconf since 2004.

AC_HELP_STRING is officially obsolete, and apparently some autoconf
versions complain about it.

My version of autoconf thinks they're functionally exactly equivalent,
but since googling AS_HELP_STRING takes you to the correct manual page
and AC_HELP_STRING doesn't, we might as well use the former.