silence warnings under macOS for cc, clang and gcc

Used the following script to check for warnings
not silenced via ${CSILENCE}:

```sh
find ???? -mindepth 1 -maxdepth 1 -type d | while read dir; do
(cd "$dir"; echo =-=-= "$dir" =-=-= ; csilence -v 1 clobber all ; echo ; csilence -v 1 clobber alt)
done 2>&1 | less
```

2012/deckmyn/Makefile

@@ -85,7 +85,8 @@ CC= cc
 ifeq ($(CC),clang)
 #
 CSILENCE+= -Wno-cast-align -Wno-comma -Wno-implicit-int-conversion \
-	-Wno-poison-system-directorie
+	-Wno-poison-system-directorie -Wno-unknown-warning-option \
+	-Wno-poison-system-directories
 #
 CWARN+= -Weverything
 #