CMakeLists: refactor to optionally use native libs #1648

Job	Run time
cancel_builds / Cancel Build(s)	3s
validate_python / Validate Python Code	1m 28s
create_client_artifacts / Windows Artifacts	1m 41s
validate_imported_protos / Validate Imported Proto Files	19s
create_client_artifacts / Linux Artifacts	23s
build_nilrt / NILRT Cross Compile with GCC 6.3.0	47m 29s
build_desktop / Windows Build	1h 42m 20s
build_desktop / Ubuntu Build	1h 9m 20s
run_ubuntu_system_tests	0s
run_win_system_tests	0s
create_release	0s
	3h 43m 3s

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