COMP: add_custom_command no longer takes SOURCE parameter

Fixes warning: Policy CMP0050 is not set: Disallow add_custom_command SOURCE signatures.
Run "cmake --help-policy CMP0050" for policy details.  Use the cmake_policy
command to set the policy and suppress this warning.

Change-Id: I4243dbde2a830ff903a919b2134a1b6a08b35085