Fix openssl detection on Silicon Macs

[alcaeus]

Targeting 1.21 for this change, as it could lead to a different SSL library being picked, which I wouldn't expect in a patch release. This is consistent with #1463 which was the original attempt to prefer openssl over Secure Transport and also targeted a minor version to avoid unexpected issues on a patch update.

On Silicon Macs, the paths for homebrew have changed, so if the PKG_CHECK_MODULES macro does not find openssl on MacOS and no OPENSSL_DIR option has been provided, it usually won't be found. As a result, Secure Transport will be picked in those circumstances.

On Silicon platforms, homebrew installs openssl into /opt/homebrew/opt/openssl, so adding that path to PHP_OPENSSL_DIR fixes the issue.

To make debugging easier, I also added the selected SSL library to the configure output.

NB: I checked whether pkg-config is available on my system, and it isn't by default. A quick search through homebrew also didn't reveal it, so my assumption is that pkg-config won't be available on the typical developer machine, hence this workaround.

[jmikola]

pkgconf is the Homebrew package for pkg-config. We already note that pkg-config is a requirement in the PECL install docs, but we could certainly repeat this for Homebrew and link to the specific package.

I'm not sure how the PHP formulae detect OpenSSL without pkg-config, since PHP itself has used it exclusively since 7.4 (see: php/php-src@fe8fdfa3). That was the point where PHP removed all of the legacy path detection that we still have as a fallback. But if Homebrew is all binaries, I suppose pkg-config might only be a requirement for the package maintainer(s) -- in contrast to PECL, where users are compiling the extension themselves.