[Cin]3 more patches for configure.ac, build/prep scripts

First adding checks for libavcodec/libavformat/libswscale/libavutil to configure's output. No automatic action happen if they missing, but at least it should be easier to spot them missing before make for unbundled builds Second patch add slowroll to suse section of bld_preoare.sh, adds two [[ ]] instead of single [ ] in its section, moves comment about Arch one line up as comment with # Third patch adding CONFIG_OPTIONS env. variable support to bld_appimage.sh so you hopefully can add your params without editing script itself (not really problem, just git will complain on merge if this file changes upstream and you do git pull)