Fix linking to older OpenSSL for Ruby < 2.7.7

[mislav]

Explicitly set PKG_CONFIG_PATH when --with-openssl-dir is used for older Rubies. Otherwise, Ruby will attempt to link to the latest OpenSSL found by pkg-config, which could be incompatible.

• Failing to install 2.7.3 on MacOS #2320 (reply in thread)
• Old Ruby, mismatched OpenSSL #2300 (comment)

Even though these Ruby versions are EOL, people are still inexplicably using them, and this problem comes up over and over again in user support questions.

# openssl gem version that fixes this is 2.1.4
$ curl -fsSL https://cache.ruby-lang.org/pub/ruby/2.7/ruby-2.7.6.tar.bz2 | \
    tar jxO -- \*/ext/openssl/openssl.gemspec | grep -w version
  spec.version       = "2.1.3"

[eregon]

Nice, PKG_CONFIG_PATH is what truffleruby internally used to set as well before it was fixed in the openssl gem.