message: Add option to build with std headers.
  If doctest forward declaration differs from standard library implementation
  in cross-compilation SDK build of doctest for the target fails.
diff: @@ -14,6 +14,7 @@ project(doctest VERSION ${ver} LANGUAGES CXX)
  option(DOCTEST_WITH_TESTS "Build tests/examples" ON)
  option(DOCTEST_WITH_MAIN_IN_STATIC_LIB "Build a static lib (cmake target) with a default main entry point" ON)
  option(DOCTEST_NO_INSTALL "Skip the installation process" OFF)
  +option(DOCTEST_USE_STD_HEADERS "Use std headers" OFF)
  add_library(${PROJECT_NAME} INTERFACE)
  add_library(${PROJECT_NAME}::${PROJECT_NAME} ALIAS ${PROJECT_NAME})
  @@ -50,6 +51,10 @@ if(DEFINED DOCTEST_THREAD_LOCAL)
  DOCTEST_THREAD_LOCAL=${DOCTEST_THREAD_LOCAL})
  endif()
  +if(DOCTEST_USE_STD_HEADERS)
  + target_compile_definitions(${PROJECT_NAME} INTERFACE DOCTEST_CONFIG_USE_STD_HEADERS)
  +endif()
  +
  ################################################################################
  ## TESTS/EXAMPLES/HELPERS
  ################################################################################
  