d0a829e9632379e42f0be5c554e3b692f0d14a95 build: fix mutex detection when building bdb on macOS (fanquake)
Pull request description:
Starting with the Apple Clang shipped with Xcode 12, [Apple has enabled -Werror=implicit-function-declaration by default](https://developer.apple.com/documentation/xcode-release-notes/xcode-12-release-notes):
> Clang now reports an error when you use a function without an explicit declaration when building C or Objective-C code for macOS (-Werror=implicit-function-declaration flag is on). This additional error detection unifies Clang’s behavior for iOS/tvOS and macOS 64-bit targets for this diagnostic. (49917738)
This causes bdbs mutex detection to fail when building on macOS (not cross-compiling):
```bash
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:
```bash
configure:18704: checking for mutexes
configure:18815: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -mmacosx-version-min=10.12 --sysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -o conftest -pipe -O2 -I/Users/michael/github/fanquake-bitcoin/depends/x86_64-apple-darwin19.6.0/include -L/Users/michael/github/fanquake-bitcoin/depends/x86_64-apple-darwin19.6.0/lib conftest.c -lpthread >&5
conftest.c:46:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
main() {
^
conftest.c:51:2: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
exit (
^
conftest.c:51:2: note: include the header <stdlib.h> or explicitly provide a declaration for 'exit'
1 warning and 1 error generated.
```
Append `-Wno-error=implicit-function-declaration` to `cflags` so that `-Wimplicit-function-declaration` [returns to being a warning](https://clang.llvm.org/docs/UsersManual.html#cmdoption-wno-error), and the configure checks succeed.
Fixes#19411.
ACKs for top commit:
laanwj:
Code review ACK d0a829e9632379e42f0be5c554e3b692f0d14a95
Tree-SHA512: 7813005b1fc0b370f843b6c0672acab32c999416e92c3f02b75d866e9c7aa41fe5822704fc74de6b65f0d7d94f2cdd05cc7c3ee83295ff1ecbc71d8492b9a2bf
* build: Enable C++11 build, require C++11 compiler
Implements #6211.
* depends: use c++11
* build: update ax_cxx_compile_stdcxx to serial 4
* build: force a c++ standard to be specified
Newer compilers may switch to newer standards by default. For example, gcc6
uses std=gnu++14 by default.
* c++11: fix libbdb build against libc++ in c++11 mode
atomic_init clashes with
* c++11: CAccountingEntry must be defined before use in a list
c++11ism. This fixes builds against libc++.
