build: improve builtin_clz* detection

The way we currently test with AC_CHECK_DECLS do not work with Clang:
```bash
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 also removes the __builtin_clz() check, as we don't actually use
it anywhere, and it's trvial to re-add detection if we do start using
it at some point.

0 files changed, 0 insertions, 0 deletions