And change install_deps.sh to build shared (instead of static) FFMPEG libraries, for consistency with other library dependencies and to simplify the logic. This may require users of install_deps.sh to rebuild FFMPEG. This is the last step that lets us get rid of LIBPATH variables and link_directories() entirely, as recommended by the CMake docs. Some fixes were needed in the find FFMPEG module to make it actually work, this code was unused up to now. Followup to D8855. Differential Revision: https://developer.blender.org/D9177