silence compiler warnings for macOS

After silencing compiler warnings for RHEL 9 Linux,
we silence the result on macOS for
Apple clang version 15.0.0 (clang-1500.3.9.4).

1996/westley/Makefile

@@ -40,7 +40,8 @@ include ../../var.mk
 #
 CSILENCE= -Wno-char-subscripts -Wno-error -Wno-implicit-function-declaration \
 	  -Wno-int-to-pointer-cast -Wno-empty-body -Wno-deprecated-non-prototype\
-	  -Wno-implicit-int -Wno-missing-parameter-type -Wno-return-type
+	  -Wno-implicit-int -Wno-missing-parameter-type -Wno-return-type \
+	  -Wno-unknown-warning-option
 
 # Common C compiler warning flags
 #