Add a empty mock "soundtouch_config.h" file and remove "CMAKE" compiler
definition that was used in #ifdef that skipped including
"soundtouch_config.h" in cmake build.
This is to avoid errors about missing include file when not using
autotools build.
Also update version to 2.3.1
Signed-off-by: Olli <oparviai'at'iki.fi>
