I'm surprised that gcc gets invoked on a make clean; are you sure that's what's generating the warning?
If it's gcc, you can pass an option like -Wno-something
or -Wnone. See your gcc manual; I don't seem to have that particular warning.
( By the way, I assume you mean version 9.0 of your distribution, not 9.0 of Linux itself (the kernel), as the latest major version of Linux is still only 2.6 )