extconf.rb - don't use pkg_config('openssl') if '--with-openssl-dir' is used

[MSP-Greg]

Description

If installing Puma with --with-openssl-dir, pkg_config('openssl') may locate the system OpenSSL, which is not desirable.

Hence, only use pkg_config('openssl') when --with-openssl-dir is not used.

See ruby/openssl#486

This problem has generated a few issues with macOS. If anyone with multiple OpenSSL installs (and a Ruby that is not using the system OpenSSL), can check this, it would be appreciated.

Note that this PR only affects using --with-openssl-dir with a Puma install.

Closes #2839. Not.sure

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[MSP-Greg]

Failed on the test skip added in PR #2883. Or, truffleruby fails, but truffleruby-head passes.