Explicitly define CMAKE_INSTALL_CMAKEDIR to work around FTBFS for now.
    
    Calamares uses plain GNUInstallDirs, but includes a submodule based on
    kde-partitionmanager which uses ECM (extra-cmake-modules). With
    extra-cmake-modules 1.6.0, there appears to be some bad interaction
    which leads to CMAKE_INSTALL_LIBDIR being empty on i686 at the line
    of CMakeLists.txt where CMAKE_INSTALL_CMAKEDIR is defined in terms of
    it.