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
```

2011/richards/Makefile

@@ -38,7 +38,8 @@ include ../../var.mk
 
 # Common C compiler warnings to silence
 #
-CSILENCE= -Wno-error -Wno-implicit-function-declaration -Wno-parentheses
+CSILENCE= -Wno-error -Wno-implicit-function-declaration -Wno-parentheses \
+	-Wno-incompatible-pointer-types
 
 # Common C compiler warning flags
 #
@@ -86,7 +87,7 @@ ifeq ($(CC),clang)
 CSILENCE+= -Wno-cast-align -Wno-missing-prototypes -Wno-missing-variable-declarations \
 	-Wno-pedantic -Wno-poison-system-directories -Wno-shadow \
 	-Wno-shorten-64-to-32 -Wno-sign-conversion -Wno-strict-prototypes \
-	-Wno-undef
+	-Wno-undef -Wno-alloca
 #
 CWARN+= -Weverything
 #