Only use ASIO_INCLUDE_DIR if it is set
Only use ASIO_INCLUDE_DIR if it is set

CMake would complain if the ASIO_INCLDUE_DIR variable was not set, but
used as include directory.
So only use it when it really is set.

tkemmer/ball,tkemmer/ball,tkemmer/ball,tkemmer/ball,tkemmer/ball,tkemmer/ball