install h.264 plugin for linphone on ubuntu

h.264 plugin isn't a standard part of linphone installation on ubuntu. We must manually compile and install it.

1. Download msx264 plugin source code.
2. Run sudo apt-get install libmediastreamer-dev libx264-dev libavcodec-dev libswscale-dev libtheora-dev to meet msx264's dependency requirements.
3. Run ./configure --prefix=/usr/lib. It's mandatory to set prefix to /usr/lib, because linphone can't find the plugin if it's installed in default location, /usr/local/lib.
4. Run sudo make install.
5. Change the line 393 in src/msx264.c from

393     if (sws_scale(s->sws_ctx,(uint8_t * const*)orig->data,orig->linesize, 0,

to

393     if (sws_scale(s->sws_ctx,(uint8_t **)orig->data,orig->linesize, 0,

to get rid of compilation error, if you have.

6. Restart linphone, the h.264 plugin should be available now.