Add vorbis dependencies when building examples.
The decoder_example program calls floor() which requires its own -lm on the link line to resolve the local reference. It seems on most platforms this was shadowed by the transitive dependency in the vorbis libraries, but it results in unddefine reference error on e.g. debian mipsel.
example-libm in 1 minute and 52 seconds (queued for 2 seconds)2 jobs for