Added support for building in a separate directory

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>
autogen.sh : Fix message.
src/xi.c : Expose string info for application and title.

Based on a patch from Stefan Sauer <ensonic@users.sf.net>.

Closes: https://github.com/erikd/libsndfile/pull/53