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_CRYPTO_COMMON_H
#define BITCOIN_CRYPTO_COMMON_H
#if defined(HAVE_CONFIG_H)
#include <config/bitcoin-config.h>
#endif
#include <stdint.h>
#include <string.h>
#include <compat/endian.h>
uint16_t static inline ReadLE16(const unsigned char* ptr)
{
    uint16_t x;
    memcpy((char*)&x, ptr, 2);
    return le16toh(x);
}
uint32_t static inline ReadLE32(const unsigned char* ptr)
{
    uint32_t x;
    memcpy((char*)&x, ptr, 4);
    return le32toh(x);
}
uint64_t static inline ReadLE64(const unsigned char* ptr)
{
    uint64_t x;
    memcpy((char*)&x, ptr, 8);
    return le64toh(x);
}
void static inline WriteLE16(unsigned char* ptr, uint16_t x)