x265 is free software library and application for encoding video
streams into the H.265/MPEG-H HEVC compression format.

This is an outdated version, fit for 15.0's default
gst-plugins-bad-nonfree

Activate this version by using
export\
PKG_CONFIG_PATH="/opt/x265-legacy3.6-opt/lib64/pkgconfig/:$PKG_CONFIG_PATH"
in your SlackBuilds, but try to port to the recent x265 if possible.
