build (macos): do not overwrite PKG_CONFIG_PATH variable

[botantony]

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macOS configure script overwrites user provided PKG_CONFIG_PATH instead of appending to it

fixes Homebrew/homebrew-core/pull/210080

[rustyrussell]

Ack 61c5582 pending @sangbida 's review...

[sangbida]

It compiles fine on my machine, although I would've liked a better PR description or commit message. The PR that this points to in homebrew seems to be a closed PR. I would've liked to replicate the issue this PR is solving and see what it fixes.