Some changes to buildsystem files

[Stephen Kelly]

Hello,

I've just pushed some changes to the buildsystem files. KF5 CMake config 
files no longer define a Foo_LIBRARIES variable. Instead the KF5::Foo should 
be used in target_link_libraries.

I tested the build of plasma-frameworks, kactivities, konsole and kde-
workspace after my changes and they built for me. If they don't build for 
you then replace those variables with target names or post here for help.

Thanks,

Steve.