Festival changes ported from Debian

 * Add PulseAudio support

 * Rewrite text2wave to avoid creating temporal files (requires EST-11).
   Current text2wave status may damage SSD disk drives due to excessive
   disk i/o. Additionally this patch improves dramatically the text2wave
   performance on long texts (audiobooks) in CPU, memory usage and disk
   i/o.

 * Improve HTS performance by adding a cache that avoids loading and
   freeing HTS models on every utterance. Cache can be disabled at will
   for "lab purposes" :-).

 * Improve HTS performance (2): mgc and f0 parameters were saved to disk
   and then deleted on every utterance. New default is: Don't create
   them. The new default is easy to override.

 * Add hts21_engine module. This is the older hts_engine module, which
   we use to maintain backwards compatibility. It is still quite
   popular.

 * Edit lib/languages.scm in order to detect new language support in the
   same way new available voices are detected. By doing this, users do
   not need to edit by hand lib/languages.scm to add support for a new
   language. Very useful in Debian.

 * Update configure.in to remove deprecated autoconf functions.