BUILD: Link against libogg when FLAC is used but not libvorbis.

The old code oddly linked against libogg when vorbis was enabled. However, in
this case libogg is already linked against. Since FLAC requires libogg we
should also link against it even when vorbis is disabled. This should fix
static libFLAC linking when libvorbis is disabled.

It might be noteworthy that statically linking against libFLAC when it was build with libogg support disabled worked before and should be broken now when libogg is not present at all... I am however not sure how we can detect that libFLAC depends on libogg easily. If anybody has some suggestions, that's most welcome.