portutil: Only warn about system headers not being installed with Xcode older than 9.3.
portutil: Only warn about system headers not being installed with Xcode older than 9.3.

Signed-off-by: Jeremy Huddleston Sequoia <db3cec0cba09acb9cf8fd8bd399a3b27498cf883@macports.org>

neverpanic/macports-base,macports/macports-base,macports/macports-base,danchr/macports-base,neverpanic/macports-base,danchr/macports-base