Separation of PJMEDIA specific implementation to support third party media stack/library in PJSUA-LIB

[bennylp]

This ticket implements support for integrating third party media stack into PJSUA-LIB. Follow these steps to integrate third party media library with PJSUA-LIB:

1. Declare this in pjlib/include/pj/config_site.h:

   #define PJSUA_MEDIA_HAS_PJMEDIA    0
   

   to exclude PJMEDIA specific implementation from PJSUA-LIB library. Understandably you will loose all media features in PJSUA-LIB (this will be handled by your third party media stack).
2. Also copy suggested settings from pjsip-apps/src/third_party_media/config_site.h into pjlib/include/pj/config_site.h. These settings are mostly used to exclude unneeded media components from the link process.
3. Build the libraries, but this time using

   $ make lib
   

   instead of just make or make all. This is because most samples will no longer build due to missing media in PJSUA-LIB, hence normal make will fail on these apps. The make lib command only builds the libraries and unit tests for the libraries.
4. Go to directory pjsip-apps/src/third_party. This is a sample application with hook points to integrate third party media library. Fill in the media implementation in the alt_pjsua_xxx.c files, following the "TODO" notes. Run make to build the application. Once it's built, run alt_pjsua just as you run the usual pjsua application (it's essentially the same app!).