Test on macOS 12.2.1 against the master branch. #24397 doesn't fix it.

```
brew info boost
boost: stable 1.78.0 (bottled), HEAD
...
./configure --enable-external-signer
...
checking for boostlib >= 1.64.0 (106400)... yes
checking whether Boost.Process can be used... no
configure: error: External signing is not supported for this Boost version
```

It does work with boost 1.76 (there was no 1.77 release in home-brew):

```
brew install boost@1.76
...
LDFLAGS="-L/usr/local/opt/boost@1.76/lib" CPPFLAGS="-I/usr/local/opt/boost@1.76/include" ./configure --enable-external-signer
...
checking for boostlib >= 1.64.0 (106400)... yes
checking whether Boost.Process can be used... yes
...
Options used to compile and link:
  external signer = yes
```

There's nothing in the release notes about Boost.Process for either [1.77](https://www.boost.org/users/history/version_1_77_0.html) or [1.78](https://www.boost.org/users/history/version_1_78_0.html).

The Homebrew [formula changes](https://github.com/Homebrew/homebrew-core/commits/master/Formula/boost.rb) are nothing but a trivial version and hash bump.
{
    constexpr uint64_t min_disk_space = 52428800; 
    uint64_t free_bytes_available = fs::space(dir).available;
    return free_bytes_available >= min_disk_space + additional_bytes;
}
std::streampos GetFileSize(const char* path, std::streamsize max) {
    std::ifstream file{path, std::ios::binary};
    file.ignore(max);
    return file.gcount();
}
/**
 * Interpret a string argument as a boolean.
 *
 * The definition of LocaleIndependentAtoi<int>() requires that non-numeric string values
 * like "foo", return 0. This means that if a user unintentionally supplies a
 * non-integer argument here, the return value is always false. This means that
 * -foo=false does what the user probably expects, but -foo=true is well defined
 * but does not do what they probably expected.
 *
 * The return value of LocaleIndependentAtoi<int>(...) is zero when given input not
 * representable as an int.
 *
 * For a more extensive discussion of this topic (and a wide range of opinions
 * on the Right Way to change this code), see PR12713.
 */
static bool InterpretBool(const std::string& strValue)