See this arbitrary macOS Travis job: https://travis-ci.org/github/bitcoin/bitcoin/jobs/702283632#L521

Somehow `AC_CHECK_DECLS` doesn't work for builtins for clang (it does work for GCC).

However, `__builtin_prefetch` (which is detected using `AC_COMPILE_IFELSE`) does work.
#define HAVE_BOOST_SYSTEM 
#define HAVE_BOOST_THREAD 
#define HAVE_BOOST_UNIT_TEST_FRAMEWORK 
#define HAVE_CONSENSUS_LIB 1
#define HAVE_CXX11 1
/* Define to 1 if you have the declaration of `be16toh', and to 0 if you
   don't. */
#define HAVE_DECL_BE16TOH 0
/* Define to 1 if you have the declaration of `be32toh', and to 0 if you
   don't. */
#define HAVE_DECL_BE32TOH 0
/* Define to 1 if you have the declaration of `be64toh', and to 0 if you
   don't. */
#define HAVE_DECL_BE64TOH 0
/* Define to 1 if you have the declaration of `bswap_16', and to 0 if you
   don't. */
#define HAVE_DECL_BSWAP_16 0
/* Define to 1 if you have the declaration of `bswap_32', and to 0 if you
   don't. */
#define HAVE_DECL_BSWAP_32 0
/* Define to 1 if you have the declaration of `bswap_64', and to 0 if you
   don't. */
#define HAVE_DECL_BSWAP_64 0
/* Define to 1 if you have the declaration of `daemon', and to 0 if you don't.
   */
#define HAVE_DECL_DAEMON 0
/* Define to 1 if you have the declaration of `htobe16', and to 0 if you
   don't. */
#define HAVE_DECL_HTOBE16 0
/* Define to 1 if you have the declaration of `htobe32', and to 0 if you