conditionally install xapian-ruby when not on OpenBSD
add explanation for extension used for OpenBSD conditional dependency

expand comment on OpenBSD installation

remove note about adding OpenBSD installation steps
don't load libc and do setlocale() on OpenBSD
xapian: correctly include extension, only install xapian when not installed.

It seems that the way the extension was included prevented it from being
run properly.

Also, xapian was always being installed on platforms where it should be
- but also when it was already installed. Now testing if exists before
installing.
add a post-install message for OpenBSD users
gemfile: specify xapian-ruby dep independently if not on openbsd, since bundler doesn't seem to get it from the extensions
gemspec: move post-install message together with the rest, otherwise it overwrites the original message.
openbsd: history and rel-notes