o*: migrate to pkgconf

[cho-m]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

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[cho-m]

Will need at least one more re-run for Linux.

  /usr/bin/ld: warning: libxmlsec1-openssl.so.1, needed by /home/linuxbrew/.linuxbrew/lib/libpskc.so, not found (try using -rpath or -rpath-link)
  /usr/bin/ld: warning: libxmlsec1.so.1, needed by /home/linuxbrew/.linuxbrew/lib/libpskc.so, not found (try using -rpath or -rpath-link)
  /usr/bin/ld: /home/linuxbrew/.linuxbrew/lib/libpskc.so: undefined reference to `xmlSecOpenSSLTransformRsaSha1GetKlass'