resolve dependency problem between RIGID and USER-OMP package

    # detects styles which have USER-OMP version

    RegisterStylesExt(${USER-OMP_SOURCES_DIR} omp OMP_SOURCES)

    get_property(USER-OMP_SOURCES GLOBAL PROPERTY OMP_SOURCES)

    # manually add classes, that are not styles

    if(PKG_RIGID)

      list(APPEND USER-OMP_SOURCES ${USER-OMP_SOURCES_DIR}/fix_rigid_nh_omp.cpp)

    endif()

    list(APPEND LIB_SOURCES ${USER-OMP_SOURCES})

    include_directories(${USER-OMP_SOURCES_DIR})

endif()