Fix build for pre-ECM CMake install.

The CI infrastructure started failing kdesrc-build after commit
864aa9d45e1987e90a140f0dd6630311d8ec2539, which I suspect is because the
CI builds and installs kdesrc-build without setting up a KDE buildsystem
(i.e. extra-cmake-modules).

This is supported in kdesrc-build, but the commit that broke the build
adds a reference a variable defined in ECM's KDEInstallDirs macro.

Fixed by adding to the existing compat layer machinery in the existing
CMakeLists.txt (at least, I hope!)