silence most warnings under macOS

For macOS Apple clang version 15.0.0 (clang-1500.3.9.4)
compiler, we have silenced the warnings that can be silenced.

We also updated `.gitignore` for `2014/maffiodo2` and `1995/vanschnitz`.

2020/ferguson2/Makefile

@@ -85,7 +85,7 @@ ifeq "$(findstring $(CLANG),${CC})" "$(CLANG)"
 #
 CSILENCE+= -Wno-implicit-int-conversion -Wno-missing-prototypes \
 	-Wno-missing-variable-declarations -Wno-poison-system-directories \
-	-Wno-shorten-64-to-32
+	-Wno-shorten-64-to-32 -Wno-shadow
 #
 CWARN+= -Weverything
 #