message: add alias target for doctest for use in build tree
diff: @@ -12,6 +12,7 @@ option(DOCTEST_WITH_MAIN_IN_STATIC_LIB "Build a static lib (cmake target) with
  option(DOCTEST_NO_INSTALL "Skip the installation process" OFF)
  add_library(${PROJECT_NAME} INTERFACE)
  +add_library(${PROJECT_NAME}::${PROJECT_NAME} ALIAS ${PROJECT_NAME})
  set(doctest_parts_folder "${CMAKE_CURRENT_SOURCE_DIR}/doctest/parts")
  