SPMUtility: limit private method to `macOS`
SPMUtility: limit private method to `macOS`

This private method is only used on macOS targets, but was globally
defined.  This depends on `confstr` which is part of the C library on
Unix systems.  However, it is not available on Windows.  Limit the
function to the platform where it is used.

abertelrud/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager