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
```

1989/ovdluhe/Makefile

@@ -40,7 +40,8 @@ include ../../var.mk
 #
 CSILENCE= -Wno-char-subscripts -Wno-constant-conversion -Wno-empty-body \
 	-Wno-implicit-function-declaration -Wno-parentheses -Wno-unsequenced \
-	-Wno-unused-variable
+	-Wno-unused-variable -Wno-strict-prototypes \
+	-Wno-extra-semi -Wno-non-literal-null-conversion
 
 # Common C compiler warning flags
 #