Albert Chin's patch to libpcap's configure.in:

	AC_CHECK_DECLS will define the HAVE_DECL_xxx variable whether it
	succeeds or fails, so you can't erase a "no" result and try
	again with a different header.  Use AC_CHECK_DECL, which doesn't
	define HAVE_DECL_xxx, and then explicitly define it based on
	whether we found a declaration or not.

is required for tcpdump's configure.in as well.