ispell.el (ispell-region): Do not prefix sent string by comment in autoconf mode
This commit changes some of the `in-comment' ocurrences to more specific
`add-comment', set only if comment prefix is to be added.
When in comment, old code prefixed string by comment string to let ispell
know this is part of a comment string.
This is only supported in some modes. In particular, this is not supported in
autoconf mode where adding the comment string messes everything up because
ispell tries to spellcheck the `dnl' string header causing misalignments in
some cases (debbugs.gnu.org: #12768).