[noetic-devel][Windows] Fix file COPY cannot read symlink

[seanyen]

A recent change from #1100 introduced an usage of file(COPY) and unfortunately this idiom has some limitation on Windows. For example, CMake doesn't support the case where the source file is a symlink, due to the absent of ReadSymlink implementation on Windows.

And when the conditions are met, it will render as errors like below:

-- rospy_tutorials: 2 messages, 2 services
CMake Error at C:/opt/ros/melodic/x64/share/catkin/cmake/catkin_install_python.cmake:45 (file):
  file COPY cannot read symlink
  "C:/workspace/rospy_ws/src/ros_tutorials/rospy_tutorials/001_talker_listener/listener"
  to duplicate at
  "C:/workspace/rospy_ws/build_isolated/rospy_tutorials/catkin_generated/installspace/listener":
  No error.
Call Stack (most recent call first):
  CMakeLists.txt:14 (catkin_install_python)

This pull request is to make the file copying conditional and unblock any build breaks due to the limitation. And please consider to back-port to kinetic-devel as well.

[dirk-thomas]

Thanks for the patch and the quick response.

[dirk-thomas]

Cherry-picked to kinetic-devel in 232339e.