Add support for pkg-config

[baarde]

When pkg-config is installed and finds the libraries, use those rather than relying on Homebrew or bundling OpenSSL.

Fixes #2544

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A few comments about package managers

• I've tried hard not to break things for Homebrew. When the libraries and pkg-config have been installed using Homebrew, pkg-config finds the libraries in /opt/homebrew/Cellar/. However, those have their version number in the path, so I decided to ignore any library in */Cellar/* to avoid breaking Ruby when running brew upgrade.
• When the libraries and pkg-config have been installed using different package managers, whoever comes first in the search path wins.
• A package manager that distributes ruby-build should have pkg-config as a dependency as well as all required and optional libraries (openssl, readline, libffi, gmp) to make sure all the linked libraries come from the same source.

Alternatives considered

Skip --with-xyz-dir=

pkg-config is used internally by Ruby build scripts. So my initial idea was to skip the --with-xyz-dir= option when a library is discovered by pkg-config and let the build scripts do their job. However:

• That doesn't work for gmp.
• That may not work for other libraries with older Rubies.
• Having the --with-xyz-dir= options appear in the output is helpful to make sure that the correct libraries are used.
• "Explicit is better than implicit." 😉

Do nothing

My goal was to write a simple fix to have ruby-build work well with MacPorts (and other package managers). But doing so without breaking things turned out a little more complex than anticipated. Maybe it's just not worth it.