build: fix mutex detection when building bdb on macOS

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.

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