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).

1988/isaak/Makefile

@@ -43,7 +43,8 @@ CSILENCE= -Wno-comma -Wno-implicit-function-declaration -Wno-format-insufficient
 	  -Wno-missing-variable-declarations -Wno-poison-system-directories -Wno-return-type \
 	  -Wno-shadow -Wno-shift-op-parentheses -Wno-strict-prototypes -Wno-unreachable-code \
 	  -Wno-unused-macros -Wno-int-conversion -Wno-macro-redefined -Wno-parentheses \
-	  -Wno-builtin-declaration-mismatch -Wno-format -Wno-missing-parameter-type
+	  -Wno-builtin-declaration-mismatch -Wno-format -Wno-missing-parameter-type \
+	  -Wno-unknown-warning-option
 
 # Common C compiler warning flags
 #