Visual Studio dependencies issue

CMakeLists.txt

cmake_minimum_required(VERSION 3.4)

project(foo)

add_custom_target(
  gen
  SOURCES "${CMAKE_CURRENT_BINARY_DIR}/lib.cpp"
  COMMENT "Custom target"
)

add_custom_command(
  OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/lib.cpp"
  COMMAND "python" "-c" "import time; print('// ' + str(time.time()))" ">" "${CMAKE_CURRENT_BINARY_DIR}/lib.cpp"
  COMMENT "Custom command generating the source file..."
  VERBATIM
)

add_library(
  lib1 SHARED
  "${CMAKE_CURRENT_BINARY_DIR}/lib.cpp"
)
add_dependencies(
  lib1
  gen
)

add_library(
  lib2 SHARED
  "${CMAKE_CURRENT_BINARY_DIR}/lib.cpp"
)
add_dependencies(
  lib2
  gen
)