Merge #19403: build: improve __builtin_clz* detection

9952242c03fe587b5dff46a9f770e319146103bf build: improve builtin_clz* detection (fanquake)

Pull request description:

  Fixes #19402.

  The way we currently test for `__builtin_clz*` support with `AC_CHECK_DECLS` does 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. If this is controversial then I'll add a test for it as well.

ACKs for top commit:
  sipa:
    ACK 9952242c03fe587b5dff46a9f770e319146103bf
  laanwj:
    ACK 9952242c03fe587b5dff46a9f770e319146103bf

Tree-SHA512: 695abb1a694a01a25aaa483b4fffa7d598842f2ba4fe8630fbed9ce5450b915c33bf34bb16ad16a16b702dd7c91ebf49fe509a2498b9e28254fe0ec5177bbac0

0 files changed, 0 insertions, 0 deletions