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
```

2013/cable3/Makefile

@@ -43,7 +43,8 @@ CSILENCE= -Wno-bitwise-op-parentheses -Wno-bool-operation \
 	-Wno-implicit-function-declaration -Wno-implicit-int \
 	-Wno-incompatible-pointer-types -Wno-logical-not-parentheses \
 	-Wno-logical-op-parentheses -Wno-parentheses -Wno-shift-op-parentheses \
-	-Wno-sign-compare -Wno-unsequenced -Wno-unused-parameter
+	-Wno-sign-compare -Wno-unsequenced -Wno-unused-parameter \
+	-Wno-deprecated-non-prototype -Wno-return-type
 
 # Common C compiler warning flags
 #