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/yang/Makefile

@@ -87,7 +87,8 @@ ifeq ($(CC),clang)
 CSILENCE+= -Wno-c99-compat -Wno-comma -Wno-double-promotion -Wno-float-conversion \
 	-Wno-implicit-float-conversion -Wno-missing-prototypes \
 	-Wno-missing-variable-declarations -Wno-poison-system-directories \
-	-Wno-unreachable-code
+	-Wno-unreachable-code -Wno-c++98-compat -Wno-old-style-cast \
+	-Wno-zero-as-null-pointer-constant
 #
 CWARN+= -Weverything
 #