Use PKG_CONFIG_PATH from environment

[nicksieger]

What

Allows injecting $PKG_CONFIG_PATH from the environment outside the brew command, for custom gem native extension build configuration.

Why

Catalina 10.15 no longer has /usr/include and we might need to tell extconf to look elsewhere when building.

Deploy Plan

Does Platform Operations need to know anything special about this deploy? Are migrations present?

Rollback Plan

• To roll back this change, revert the merge with: git revert -m 1 MERGE_SHA and perform another deploy.

URLs

• https://people.freedesktop.org/~dbn/pkg-config-guide.html

QA Plan

Reference commonly used Regression test plans on the QA Wiki Fill in scenarios below in checklist format. Consider Regression scenarios (did we break something else related to this change) in addition to Happy Path (testing the new feature directly). Evaluate the risk level and label accordingly and ensure the QA Plan matches the risk level!

• A step to reproduce (e.g.: navigate to http://example.com/foo/bar; fill a field labeled "login:" with "boo"; click "submit"; press Enter)
• [ ] An assertion to verify (e.g. "that we are redirected to user dashboard"; "there's a message on the top of page saying 'login successful'"