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
```

2015/duble/Makefile

@@ -41,7 +41,7 @@ include ../../var.mk
 CSILENCE= -Wno-bitwise-op-parentheses -Wno-implicit-int -Wno-int-conversion \
 	    -Wno-logical-op-parentheses -Wno-overlength-strings -Wno-return-type \
 	    -Wno-shift-op-parentheses -Wno-string-plus-int -Wno-unsequenced \
-	    -Wno-unused-value
+	    -Wno-unused-value -Wno-strict-prototypes
 
 # Common C compiler warning flags
 #