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 to 1 if you have the declaration of `__builtin_clzl', and to 0 if
   you don't. */
/* Define to 1 if you have the declaration of `__builtin_clzll', and to 0 if
   you don't. */
#define HAVE_FUNC_ATTRIBUTE_DLLEXPORT 1
#define HAVE_FUNC_ATTRIBUTE_DLLIMPORT 1
#define HAVE_FUNC_ATTRIBUTE_VISIBILITY 1
/* Define this symbol if the BSD getentropy system call is available with
   sys/random.h */
#define HAVE_INTTYPES_H 1
#define HAVE_MEMORY_H 1
#define HAVE_MINIUPNPC_MINIUPNPC_H 1
#define HAVE_MINIUPNPC_MINIWGET_H 1
#define HAVE_MINIUPNPC_UPNPCOMMANDS_H 1
#define HAVE_MINIUPNPC_UPNPERRORS_H 1
#define HAVE_STDINT_H 1
#define HAVE_STDIO_H 1
#define HAVE_STDLIB_H 1
#define HAVE_STRINGS_H 1
#define HAVE_STRING_H 1
#define HAVE_SYS_STAT_H 1
#define HAVE_SYS_TYPES_H 1
#define HAVE_VISIBILITY_ATTRIBUTE 1
#define LT_OBJDIR ".libs/"
#define PACKAGE_BUGREPORT "https://github.com/bitcoin/bitcoin/issues"
#define PACKAGE_NAME "Bitcoin Core"
#define PACKAGE_STRING "Bitcoin Core 0.19.99"
#define PACKAGE_TARNAME "bitcoin"
#define PACKAGE_URL "https://bitcoincore.org/"