ogg.m4:8: warning: underquoted definition of XIPH_PATH_OGG

Automake-1.8 complains that vorbis.m4 has an underquoted definition of
XIPH_PATH_OGG. According to
<http://mail.gnu.org/archive/html/libtool-patches/2002-10/msg00031.html>, these
are known to cause problems and possibly even severer errors in the future.
Therefore, I am attaching a patch that quotes XIPH_PATH_OGG.

(This bug is related to bug 500.)