cmake_minimum_required(VERSION 2.8)
if(NOT EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/foo.cpp)
    message("Generating foo.cpp")
    file(WRITE ${CMAKE_CURRENT_SOURCE_DIR}/foo.cpp "int main() {}")
endif()
add_custom_command(OUTPUT foo2.cpp
                    COMMAND ${CMAKE_COMMAND} -E copy_if_different ${CMAKE_CURRENT_SOURCE_DIR}/foo.cpp ${CMAKE_CURRENT_BINARY_DIR}/foo2.cpp
                    DEPENDS foo.cpp)
set_source_files_properties(foo2.cpp PROPERTIES GENERATED ON)
add_executable(foo foo2.cpp)