Check if FastRTPS available in demo_nodes_cpp_native before creating library/executable (#383)
Check if FastRTPS available in demo_nodes_cpp_native before creating library/executable (#383)

* fix for when FastRTPS not available

Signed-off-by: Siddharth Kucheria <d65c5de2f9241ed00720b7a6cb488b563b037de2@usc.edu>

ros2/demos,ros2/demos,ros2/demos,ros2/demos