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
```

1990/tbr/Makefile

@@ -39,7 +39,7 @@ include ../../var.mk
 # Common C compiler warnings to silence
 #
 CSILENCE= -Wno-char-subscripts -Wno-error -Wno-implicit-function-declaration \
-	-Wno-unused-value
+	-Wno-unused-value -Wno-deprecated-non-prototype -Wno-return-type
 
 # Common C compiler warning flags
 #
@@ -85,7 +85,8 @@ CC= cc
 ifeq ($(CC),clang)
 #
 CSILENCE+= -Wno-comma -Wno-missing-prototypes -Wno-missing-variable-declarations \
-	-Wno-pedantic -Wno-poison-system-directories -Wno-strict-prototypes
+	-Wno-pedantic -Wno-poison-system-directories -Wno-strict-prototypes \
+	-Wno-unreachable-code
 #
 CWARN+= -Weverything
 #