attempt fixing zlib compilation with gcc7.2

Makefile

@@ -8372,7 +8372,7 @@ PUBLIC_HEADERS_C += \
 
 LIBZ_OBJS = $(addprefix $(OBJDIR)/$(CONFIG)/, $(addsuffix .o, $(basename $(LIBZ_SRC))))
 
-$(LIBZ_OBJS): CFLAGS += -Wno-sign-conversion -Wno-conversion -Wno-unused-value -Wno-implicit-function-declaration $(W_NO_SHIFT_NEGATIVE_VALUE) -fvisibility=hidden
+$(LIBZ_OBJS): CFLAGS += -Wno-sign-conversion -Wno-conversion -Wno-unused-value -Wno-implicit-function-declaration -Wno-implicit-fallthrough $(W_NO_SHIFT_NEGATIVE_VALUE) -fvisibility=hidden
 
 $(LIBDIR)/$(CONFIG)/libz.a: $(CARES_DEP)  $(LIBZ_OBJS) 
 	$(E) "[AR]      Creating $@"

build.yaml

@@ -5052,7 +5052,7 @@ defaults:
     LDFLAGS: -g
   zlib:
     CFLAGS: -Wno-sign-conversion -Wno-conversion -Wno-unused-value -Wno-implicit-function-declaration
-      $(W_NO_SHIFT_NEGATIVE_VALUE) -fvisibility=hidden
+      -Wno-implicit-fallthrough $(W_NO_SHIFT_NEGATIVE_VALUE) -fvisibility=hidden
 openssl_fallback:
   base_uri: https://openssl.org/source/old/1.0.2/
   extraction_dir: openssl-1.0.2f