Make AM_PATH_OGG, AM_PATH_VORBIS, and AM_PATH_AO each a bit smarter.
Each of the three will now also check the $prefix for the location
of their respective libraries/header files.  This means that if you're
compiling all 4 things (ao, ogg, vorbis, vorbis-tools) all with the
same $prefix (which is a pretty common case), you don't have to specify
"--with-ao-prefix=this --with-ogg-prefix=really --with-vorbis-prefix=sucks",
you can just specify a single --prefix argument and ditch all the
--with-*-prefix arguments.

However, if you do specify any of the three --with arguments, those are
searched *first* -- the $prefix is searched second.

svn path=/trunk/vorbis-tools/; revision=1371