Support for OpenH264 codec

[bennylp]

Support for the open source implementation of H.264 codec from ​http://www.openh264.org.

Below are the steps to use the codec:

For GNU targets:

1. Run configure with --with-openh264 option specifying the directory of OpenH264 source files. E.g.:

   ./configure --with-openh264=/home/user/src/openh264-trunk
   

2. Add this to your config_site.h:

   #define PJMEDIA_HAS_OPENH264_CODEC		1
   

For Visual Studio:

1. Add this to your config_site.h:

   #define PJMEDIA_HAS_OPENH264_CODEC		1