Add defines to exclude encoder specific parts from library
Several things can be removed from the library at compile time by using #defines if only a vorbis decoder is needed. These include, but are not limted to:
- Floor packer + Floor forward function
- Residue packer + Residue forward function
- Mapping packer + Mapping forward function
- Codebook packer/encoder
- Bitrate management (bitrate.c)
- vorbis_block_internal and functions that deal with it.
- Envelope code (envelope.c)
- Psychoaucostic model code (psy.c)
- Analysis file (analysis.c)
- info packers (info.c)
This reduces the size of the generated code from >100kb to ~60kb.