Avoid absolute path for install targets
Avoid absolute path for install targets

I get an error when packaging a project with RTK as a submodule of ITK, with ITK added via. add_subdirectory.
The project is packaged with WiX on Windows. (I haven't tested packaging on other OS's nor packagers)
This fixes the problem for me. I guess one could give an empty ´CMAKE_INSTALL_PREFIX´ at configuration, to fix it as well.
However, I don't think this fix changes/breaks the ´install´ step, where ´CMAKE_INSTALL_PREFIX´ should be automatically added by ´install()´, when given a relative path.
SimonRit/RTK,SimonRit/RTK,SimonRit/RTK,SimonRit/RTK