subdirmk: warning (unknown-warning): ./Prefix.sd.mk:2: unknown warning many-requests-for-unknown-warning requested
subdirmk: warning (single-char-var): ./Dir.sd.mk:8: possibly confusing unbracketed single-char $-expansion
subdirmk: warning (local+global): saw both WARN and &WARN
 saw WARN at ./Dir.sd.mk:6
 saw WARN at ./Dir.sd.mk:9
 saw &WARN at ./Dir.sd.mk:7
 saw &WARN at ./Dir.sd.mk:10
 saw &WARN at ./Dir.sd.mk:11
subdirmk: warning (unknown-warning): ./sub/dir/Dir.sd.mk:3: attempt to suppress unknown warning(s) `some-unknown-warning'