revert my change to use the AC_PREFIX_DEFAULT macro since it is not something

that is expanded inline, so it didn't do what I wanted it to. The --help output
will be wrong again, but that's better than completely breaking it. :)
(Thanks to jcollie for catching this!)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@35439 65c4cc65-6c06-0410-ace0-fbb531ad65f3