Provide a more sensible CMAKE_INSTALL_PREFIX default

Summary: If no prefix is passed, use ECM's by default, instead of defaulting to /usr/local which isn't optimal in any case.

Reviewers: cgiboudeaux

Reviewed By: cgiboudeaux

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

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D22676