CMake 3.20.0 introduces a new policy that allows us to specify the
behavior of the DEPFILE argument to add_custom_command() for Ninja
builds. Setting this policy to OLD makes DEPFILE be either an absolute
path or a path relative to CMAKE_BINARY_DIR. In contrast, the NEW
behavior makes the path absolute or relative to CMAKE_CURRENT_BINARY_DIR.
CMake only emits a warning because DEPFILE is used in a subdirectory. If
it were not, the build would default to the OLD behavior. Explicitly set
the policy to OLD to silence this warning for Ninja builds.
Additional information: cmake --help-policy CMP0116