COMP: Remove add_custom_command(SOURCE...

These are deprecated in future versions of CMake.

Co-authored by: Brad King <brad.king@kitware.com>

Change-Id: I49f3f3e7e1e884e86772c69bcb166dae29c53f8e
Merge topic 'custom_command-source2'

0ad45762 COMP: Remove add_custom_command(SOURCE...