ECMGeneratePkgConfigFile: make all vars dependent on ${prefix}

Summary:
Note that the first test had a comment "without optional arguments"
so I removed the INCLUDE_INSTALL_DIR variable to test what happens by
default. The other tests still pass an absolute path there (which is
unusual in practice, and breaks the idea of using ${prefix}).

Test Plan: ctest -R PkgConfig passes

Reviewers: kossebau, apol, cgiboudeaux

Reviewed By: apol

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

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D19538