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
```

2000/schneiderwent/Makefile

@@ -83,7 +83,8 @@ CC= cc
 #
 ifeq ($(CC),clang)
 #
-CSILENCE+= -Wno-missing-variable-declarations -Wno-poison-system-directories
+CSILENCE+= -Wno-missing-variable-declarations -Wno-poison-system-directories \
+	-Wno-shorten-64-to-32 -Wno-sign-conversion
 #
 CWARN+= -Weverything
 #