Blender Git Loki

To aid linking, I added NAN_USE_FFMPEG_CONFIG (true/false).
If "export NAN_USE_FFMPEG_CONFIG=true" is added to user-def.mk,
the system executes the ffmeg-config program to set values
for NAN_FFMPEG (--prefix), NAN_FFMPEGLIBS (--libs avcodec avformat),
and NAN_FFMPEGCFLAGS (--cflags). Only one used so far is the
NAN_FFMPEGLIBS for linking on linux (if requested to do so).
Current default is not to do this.