cmake_minimum_required(VERSION 3.0)
project(Foo)

set(A "${PROJECT_BINARY_DIR}/A.cpp")
set(B "${PROJECT_BINARY_DIR}/B.cpp")
set(GenScript "${CMAKE_CURRENT_LIST_DIR}/generate.cmake")

add_custom_command(
    OUTPUT "${A}" "${B}"
    COMMAND "${CMAKE_COMMAND}" "-DA=${A}" "-DB=${B}" -P "${GenScript}"
    DEPENDS "${GenScript}"
    COMMENT "Custom command"
)

add_custom_target(
    Generate
    DEPENDS "${A}" "${B}"
    COMMENT "Custom target"
)

add_library(Alib "${A}")
add_library(Blib "${B}")

add_dependencies(Alib Generate)
add_dependencies(Blib Generate)