Example for add_custom_command/target

CMakeLists.txt

cmake_minimum_required (VERSION 3.17)

project (custom_build)

function (build_foo_ var_)
    set (artifacts_)
    foreach (src_ IN LISTS ARGN)
        if (src_ MATCHES "\.fuga$")
            get_filename_component (base_ ${src_} NAME_WLE)
            set (dst_ ${CMAKE_CURRENT_BINARY_DIR}/${base_}.foo)
            add_custom_command (OUTPUT ${dst_}
                                COMMAND CMD -o ${dst_} ${src_}
                                MAIN_DEPENDENCY ${src_})
            list (APPEND artifacts_ ${dst_})
        endif ()
    endforeach ()
    set (${var_} ${artifacts_} PARENT_SCOPE)
endfunction ()

build_foo_ (generated_ a.txt b.txt c.txt d.fuga)

add_custom_target (gen_foo_ ALL
                   DEPENDS ${generated_})