CMake: Don't assume we have a "cat" command.

Use "copy /B" on Windows to create wireshark.pod, similar to
doc/Makefile.nmake.

Change-Id: I454afca0bd3497cca795ca18256dd7c4e2f224d4
Reviewed-on: https://code.wireshark.org/review/4415
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>