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
```

1998/df/Makefile

@@ -39,7 +39,8 @@ include ../../var.mk
 # Common C compiler warnings to silence
 #
 CSILENCE= -Wno-bitwise-op-parentheses -Wno-char-subscripts -Wno-pointer-to-int-cast \
-	-Wno-return-type -Wno-unused-parameter -Wno-unused-variable -Wno-typedef-redefinition
+	-Wno-return-type -Wno-unused-parameter -Wno-unused-variable \
+	-Wno-typedef-redefinition -Wno-deprecated-non-prototype
 
 # Common C compiler warning flags
 #
@@ -88,7 +89,7 @@ CSILENCE+= -Wno-array-bounds-pointer-arithmetic -Wno-implicit-int-conversion \
 	-Wno-keyword-macro -Wno-missing-prototypes \
 	-Wno-missing-variable-declarations -Wno-pedantic \
 	-Wno-poison-system-directories -Wno-shadow -Wno-sign-conversion \
-	-Wno-strict-prototypes
+	-Wno-strict-prototypes -Wno-cast-function-type
 #
 CWARN+= -Weverything
 #