on linux assume datapath is always relative to cmake_install_prefix, unless you provide an absolute path; on win32/osx hardcode engine path to default installation prefixes (it gets overridden at runtime so it works even if you move the data folder)
adjust cpack ignored files (libraries, mobile project files and win res files are still necessary)
hopefully fix
bug #523, file extension association
convert HAVE_NETSERVER to check for file existance, so that it's easier to provide missing servers