armadillo provides CMake targets:

    find_package(Armadillo CONFIG REQUIRED)
    target_link_libraries(main PRIVATE armadillo)

armadillo is compatible with built-in CMake variables:

    find_package(Armadillo REQUIRED)
    target_include_directories(main PRIVATE ${ARMADILLO_INCLUDE_DIRS})
    target_link_libraries(main PRIVATE ${ARMADILLO_LIBRARIES})