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
```

1993/jonth/Makefile

@@ -39,7 +39,10 @@ include ../../var.mk
 # Common C compiler warnings to silence
 #
 CSILENCE= -Wno-error -Wno-implicit-function-declaration \
-	-Wno-incompatible-pointer-types -Wno-int-conversion
+	-Wno-incompatible-pointer-types -Wno-int-conversion \
+	-Wno-c99-extensions -Wno-deprecated-declarations \
+        -Wno-deprecated-non-prototype -Wno-format -Wno-pedantic -Wno-return-type \
+        -Wno-strict-prototypes -Wno-unused-variable
 
 # Common C compiler warning flags
 #
@@ -87,7 +90,8 @@ ifeq ($(CC),clang)
 #
 CSILENCE+= -Wno-cast-align -Wno-comma -Wno-missing-prototypes \
 	-Wno-missing-variable-declarations -Wno-poison-system-directories \
-	-Wno-sign-conversion -Wno-strict-prototypes
+	-Wno-sign-conversion -Wno-strict-prototypes \
+	-Wno-implicit-int-conversion -Wno-shadow -Wno-shorten-64-to-32
 #
 CWARN+= -Weverything
 #