classpath: Fix breakage caused by -Werror=implicit-fallthrough

classpath builds with -Wextra and, unless configured with
--disable-werror, -Werror.  Since GCC 7 added -Wimplicit-fallthrough=3
to -Wextra we need to make it not an error for code that doesn't use
__attribute__((fallthrough)) yet.

Signed-off-by: Daniel Santos <daniel.santos@pobox.com>

libs/classpath/Makefile

@@ -50,6 +50,7 @@ define Download/antlr
 endef
 $(eval $(call Download,antlr))
 
+EXTRA_CFLAGS += -Wno-error=implicit-fallthrough
 CONFIGURE_ARGS += \
 	--with-gmp="$(STAGING_DIR)/usr" \
 	--without-x \