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 PACKAGE_VERSION "0.19.99"
/* Define to necessary symbol if this constant uses a non-standard name on
   your system. */
#define QT_QPA_PLATFORM_MINIMAL 1
#define QT_QPA_PLATFORM_WINDOWS 1
#define QT_STATICPLUGIN 1
#define STDC_HEADERS 1
/* UPnP support not compiled if undefined, otherwise value (0 or 1) determines
   default state */
/* Define WORDS_BIGENDIAN to 1 if your processor stores words with the most
   significant byte first (like Motorola and SPARC, unlike Intel). */
#if defined AC_APPLE_UNIVERSAL_BUILD
# if defined __BIG_ENDIAN__
#  define WORDS_BIGENDIAN 1
# endif
#else
# ifndef WORDS_BIGENDIAN
# endif
#endif
#ifndef _DARWIN_USE_64_BIT_INODE
# define _DARWIN_USE_64_BIT_INODE 1
#endif
#define _FILE_OFFSET_BITS 64
#if !defined(WINAPI_FAMILY) || (WINAPI_FAMILY == WINAPI_FAMILY_DESKTOP_APP)
/* Either a desktop application without API restrictions, or and older system
   before these macros were defined. */
#define HAVE_SYSTEM 1
#endif 
#endif