Adding portaudio support. Implementing it as a new audio_..._type in dsd.h.
Update CMakeLists.txt and create FindLibPortAudio.cmake
CMake portaudio check
Every build from commit 20271c6 (Apr 10 2013) is versioned as 1.6.0.
Add a build version after the version number
Digital Speech Decoder 1.6.0 (build:v1.6.0-20-g16110bd)
The build string uses git describe to list the tag closest to head,
and the number of commits past that tag, the a litteral g and ending
with the current SHA1
