_check_xcode_version improvements
_check_xcode_version improvements

Warn about missing system headers with Xcode 10.x on 10.13.
Warn about missing native SDK on 10.14 and later.

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