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
as previously emitted warnings are being turned into errors. i.e:
error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
Append -Wno-error=implicit-function-declaration to cflags so that
-Wimplicit-function-declaration returns to being a warning, and the
configure checks will succeed.
Fixes#19411.
d0a829e963