Passing "-DENABLE_THUMBNAILER=ON" to CMake causes it to install
"ffmpegthumbnailer.thumbnailer" into /share/thumbnailers, making it
actually usable with desktop environments.
Cleanups:
- remove trailing whitespace
- change pkgconfig -> pkg-config
- change license from gpl2 to gpl2Plus
After making `ffmpeg` point to the latest `ffmpeg_4`, all packages that
used `ffmpeg` without requiring a specific version now use ffmpeg_3
explicitly so they shouldn't change.