Zoneminder no longer builds when using -DZM_NO_FFMPEG=ON

[knight-of-ni]

Not surprisingly, zm_rtp_source.cpp does not compile w/o ffmpeg.

Need to do something like this to skip over this code during compile time.

if HAVE_LIBAVCODEC

...

endif

I'm out of time tonight and will get to this if someone else does not beat me to it.

[knight-of-ni]

It was easier than I thought. At least I think. Sanity check appreciated. See pull request #286