```
Starting with the Clang shipped with Xcode 12, Apple has enabled
-Werror=implicit-function-declaration by default. This causes bdbs mutex
detection to fail when building on macOS (not cross-compiling):
checking for mutexes... UNIX/fcntl
configure: WARNING: NO SHARED LATCH IMPLEMENTATION FOUND FOR THIS PLATFORM.
configure: error: Unable to find a mutex implementation
```
Backport of core#20195.