cmake - cmake + ninja + googletest + ExternalProject_Add の BUILD_BYPRODUCTS と依存関係サイクル

Question

外部プロジェクトを使用して Google テストをビルドしようとしています。

   # Add googletest
ExternalProject_Add( googletest
    GIT_REPOSITORY https://github.com/google/googletest.git

    # We don't need to run update command. Takes time
    # and the version we initially d/l will shoudl be fine
    CMAKE_ARGS = "-Dgtest_disable_pthreads=1"

    # Don't run update
    UPDATE_COMMAND ""

    # Disable install step
    INSTALL_COMMAND ""

   # BUILD_BYPRODUCTS googletest-prefix/src/googletest-stamp/googletest-gitinfo.txt
   # BUILD_BYPRODUCTS googletest-prefix/tmp/googletest-cfgcmd.txt
    BUILD_BYPRODUCTS "googletest-prefix/src/googletest-build/googlemock/libgmock_main.a"
    )
# Get include dirs for googletest framework
ExternalProject_Get_Property(googletest source_dir)
set(GTEST_INCLUDE_DIRS
   ${source_dir}/googlemock/include
   ${source_dir}/googletest/include
   )

# Create library target for gmock main, which is used to create
# test executables
ExternalProject_Get_Property(googletest binary_dir)
set(GTEST_LIBRARY_PATH ${binary_dir}/googlemock/libgmock_main.a)
set(GTEST_LIBRARY gmock_main)
add_library(${GTEST_LIBRARY} UNKNOWN IMPORTED)
set_property(TARGET ${GTEST_LIBRARY} PROPERTY IMPORTED_LOCATION ${GTEST_LIBRARY_PATH})
add_dependencies(${GTEST_LIBRARY} googletest)

忍者ジェネレーターを使用すると、以下の警告が表示されます。

 Policy CMP0058 is not set: Ninja requires custom command byproducts to be
  explicit.  Run "cmake --help-policy CMP0058" for policy details.  Use the
  cmake_policy command to set the policy and suppress this warning.

  This project specifies custom command DEPENDS on files in the build tree
  that are not specified as the OUTPUT or BYPRODUCTS of any
  add_custom_command or add_custom_target:

   googletest-prefix/src/googletest-stamp/googletest-gitinfo.txt
   googletest-prefix/tmp/googletest-cfgcmd.txt

  For compatibility with versions of CMake that did not have the BYPRODUCTS
  option, CMake is generating phony rules for such files to convince 'ninja'
  to build.

  Project authors should add the missing BYPRODUCTS or OUTPUT options to the
  custom commands that produce these files.

外部プロジェクト コマンドの build byproducts 行のコメントを外して cmake エラーの要求を受け入れると、循環依存エラーが発生します。ただし、ビルド副産物を除外すると、プロジェクトは問題なくビルドされるようです。

$ ninja
ninja: error: dependency cycle: googletest-prefix/src/googletest-stamp/googletest-configure -> googletest-prefix/tmp/googletest-cfgcmd.txt -> googletest-prefix/src/googletest-stamp/googletest-configure

私はcmake 3.4、ninja 1.6を使用しており、MSYS2パッケージを使用してWindowsで実行しています。