Libical is an Open Source implementation of the iCalendar protocols
and protocol data units.

This version installs into /opt/

To use this library in your slackbuilds instead of the default
15.0's libical-3.0.*, add the following into your config run

PKG_CONFIG_PATH=\
"/opt/libical4/lib$LIBDIRSUFFIX/pkgconfig/:$PKG_CONFIG_PATH"