In order to build LibreOffice with support for KDE5 (Plasma5), the following
set of additional packages is required.
None of them are run-time dependencies, so the packages can be built with
Plasma5 support and then installed and used on a computer without Plasma5.

The packages mentioned below can be obtained from https://slackware.nl/alien-kde/current/

Qt5 support:
libdbusmenu-qt5

Plasma5 support:
All of Frameworks.