Blender Git Loki

More fixes:
*Unset LANG envar for the script execution time, because commands like "apt-cache policy" will return translated messages, which would not be grep'ed crrectly...
*Disable using distrib's ffmpeg for DEB distribs too, as e.g. official debian features libav under ffmpeg name, which is not that much compatible with current Blender code.
*Also check for libxvidcore-dev .deb package, debian has no libxvidcore4-dev ;)