[AD] Tremor support

[Peter Wang <novalazy@xxxxxxxxxx>]

Trent,

Where did you get your copy of Tremor from?
I got it from http://wiki.xiph.org/Tremor with
svn co http://svn.xiph.org/trunk/Tremor/

After installation the files are:

    include/tremor/config_types.h
    include/tremor/ivorbiscodec.h
    include/tremor/ivorbisfile.h
    lib/libvorbisidec.a
    lib/libvorbisidec.la
    lib/libvorbisidec.so.1.0.2
    lib/pkgconfig/vorbisidec.pc

FindTremor.cmake currently looks for:

- the include directory to be named Tremor (uppercase)
- the file Tremor/ogg.h (which isn't used)
- the library to be called libTremor instead of libvorbisidec

I wonder which are the canonical names we should be looking for?
We can search for both if necessary.

I will rename USE_TREMOR to WANT_TREMOR, and move ALLEGRO_CFG_TREMOR
from alplatf.h to aintern_acodec_cfg.h.cmake.

Peter