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).

2001/westley/Makefile

@@ -42,7 +42,8 @@ CSILENCE= -Wno-empty-body -Wno-error -Wno-implicit-function-declaration \
 	  -Wno-implicit-int -Wno-main -Wno-return-type -Wno-trigraphs \
 	  -Wno-unused-parameter -Wno-incompatible-pointer-types \
 	  -Wno-deprecated-declarations -Wno-deprecated-non-prototype \
-	  -Wno-builtin-declaration-mismatch -Wno-restrict
+	  -Wno-builtin-declaration-mismatch -Wno-restrict \
+	  -Wno-unknown-warning-option
 
 # Common C compiler warning flags
 #