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.
#ifndef BITCOIN_BITCOIN_CONFIG_H
#define BITCOIN_BITCOIN_CONFIG_H
#define CLIENT_VERSION_BUILD 0
#define CLIENT_VERSION_IS_RELEASE false
#define CLIENT_VERSION_MAJOR 0
#define CLIENT_VERSION_MINOR 20
#define CLIENT_VERSION_REVISION 99
#define COPYRIGHT_HOLDERS "The %s developers"
#define COPYRIGHT_HOLDERS_FINAL "The Bitcoin Core developers"
#define COPYRIGHT_HOLDERS_SUBSTITUTION "Bitcoin Core"
#define COPYRIGHT_YEAR 2019
#define ENABLE_WALLET 1
#define ENABLE_ZMQ 1
#define HAVE_BOOST 
#define HAVE_BOOST_FILESYSTEM