Configuration option to have external sound device implementation

[bennylp]

Currently pjmedia allows selection of audio backend via build settings (the PJMEDIA_SOUND_IMPLEMENTATION macro), and with this setting one can select either of this backend implementation:

• PortAudio (the default)
• DirectSound (deprecated)
• Null device (just to let the build process go through).

This implies that new sound backend implementation must create a new configuration macro, which is not ideal since it requires modification to the source.

With the "external" option, no sound device backend will be provided in pjmedia library, so this would enable application to provide it's own sound device backend implementation (by just linking the application with the implementation).

[bennylp]

Done in r2182. Please see External_Sound_Device for more info.