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
```

2019/karns/Makefile

@@ -41,7 +41,7 @@ include ../../var.mk
 CSILENCE= -Wno-error -Wno-implicit-function-declaration -Wno-implicit-int \
 	-Wno-incompatible-function-pointer-types -Wno-logical-op-parentheses \
 	-Wno-sign-compare -Wno-tautological-bitwise-compare -Wno-unused-label \
-	-Wno-unused-variable
+	-Wno-unused-variable -Wno-strict-prototypes
 
 # Common C compiler warning flags
 #
@@ -88,7 +88,7 @@ ifeq ($(CC),clang)
 #
 CSILENCE+= -Wno-conditional-uninitialized -Wno-missing-variable-declarations \
 	-Wno-poison-system-directories -Wno-shadow -Wno-shorten-64-to-32 \
-	-Wno-sign-conversion
+	-Wno-sign-conversion -Wno-declaration-after-statement
 #
 CWARN+= -Weverything
 #