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/robison/Makefile

@@ -40,7 +40,7 @@ include ../../var.mk
 #
 CSILENCE= -Wno-bitwise-op-parentheses -Wno-error -Wno-implicit-function-declaration \
 	-Wno-format-security -Wno-parentheses -Wno-shift-negative-value \
-	-Wno-shift-op-parentheses
+	-Wno-shift-op-parentheses -Wno-strict-prototypes
 
 # Common C compiler warning flags
 #