Notify users when not using KDE_INSTALL_USE_QT_SYS_PATHS about prefix.sh

Summary:
It's especially when KDE_INSTALL_USE_QT_SYS_PATHS=OFF that the different environment variables will have to be initialised.
This adds a message that should tell users about the script so they have a hint of what is going on.

Reviewers: Frameworks, albertvaka, cgiboudeaux

Reviewed By: albertvaka, cgiboudeaux

Subscribers: cgiboudeaux, albertvaka, kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D22580