Add support for the speex codec.

Add support for the speex codec.

This patch adds support for yet another audio format: speex, a
CELP-based codec designed for speech.

As speex is usually used in combination with the OGG container format,
we only support this combination. The new OGG/Speex audio format
handler provides a callback structure for the generic ogg code,
very similar to the OGG/Vorbis audio format handler.

The new speex code is split over three source files: spx_afh.c contains
the audio format handler, spxdec_filter.c the decoder and spx_common.c
common functions used by both audio format handler and decoder.

Many thanks to Jean-Marc Valin, the author of the reference
implementation of the speex decoder. Reusing parts of his code made
it easy to support speex within paraslash.