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.
From config.log:
configure:21492: clang++-10 -std=c++11 -c -g -O2 -DHAVE_BUILD_INFO -D__STDC_FORMAT_MACROS conftest.cpp >&5
conftest.cpp:100:10: error: builtin functions must be directly called
(void) __builtin_clz;
^
1 error generated.
This is not specific to macOS, the configure checks currently fail with both LLVM and Apple Clang.
Yes, I detected it on Linux when building with clang.