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.
don't. */
#define HAVE_DECL_HTOBE32 0
/* Define to 1 if you have the declaration of `htobe64', and to 0 if you
   don't. */
#define HAVE_DECL_HTOBE64 0
/* Define to 1 if you have the declaration of `htole16', and to 0 if you
   don't. */
#define HAVE_DECL_HTOLE16 0
/* Define to 1 if you have the declaration of `htole32', and to 0 if you
   don't. */
#define HAVE_DECL_HTOLE32 0
/* Define to 1 if you have the declaration of `htole64', and to 0 if you
   don't. */
#define HAVE_DECL_HTOLE64 0
/* Define to 1 if you have the declaration of `le16toh', and to 0 if you
   don't. */
#define HAVE_DECL_LE16TOH 0
/* Define to 1 if you have the declaration of `le32toh', and to 0 if you
   don't. */
#define HAVE_DECL_LE32TOH 0
/* Define to 1 if you have the declaration of `le64toh', and to 0 if you
   don't. */
#define HAVE_DECL_LE64TOH 0
/* Define to 1 if you have the declaration of `strerror_r', and to 0 if you
   don't. */
#define HAVE_DECL_STRERROR_R 0
/* Define to 1 if you have the declaration of `strnlen', and to 0 if you
   don't. */
#define HAVE_DECL_STRNLEN 1
/* Define to 1 if you have the declaration of `__builtin_clz', and to 0 if you