Qt 5.11 was downgraded to 5.9 because of two issues:
- spawns errors like
```
qrc:/qml/SignInWaiting.qml:20:9: QML BusyIndicator: Detected anchors on an item that is managed by a layout. This is undefined behavior; use Layout.alignment instead.
qrc:/qml/SignInWaiting.qml:26:9: QML Text: Detected anchors on an item that is managed by a layout. This is undefined behavior; use Layout.alignment instead.
qrc:/qml/SignInWaiting.qml:20:9: QML BusyIndicator: Detected anchors on an item that is managed by a layout. This is undefined behavior; use Layout.alignment instead.
qrc:/qml/SignInWaiting.qml:26:9: QML Text: Detected anchors on an item that is managed by a layout. This is undefined behavior; use Layout.alignment instead.
qrc:/qml/SignInWaiting.qml:20:9: QML BusyIndicator: Detected anchors on an item that is managed by a layout. This is undefined behavior; use Layout.alignment instead.
qrc:/qml/SignInWaiting.qml:26:9: QML Text: Detected anchors on an item that is managed by a layout. This is undefined behavior; use Layout.alignment instead.
qrc:/qml/SignInWaiting.qml:20:9: QML BusyIndicator: Detected anchors on an item that is managed by a layout. This is undefined behavior; use Layout.alignment instead.
qrc:/qml/SignInWaiting.qml:26:9: QML Text: Detected anchors on an item that is managed by a layout. This is undefined behavior; use Layout.alignment instead.
qrc:/qml/SignInWaiting.qml:20:9: QML BusyIndicator: Detected anchors on an item that is managed by a layout. This is undefined behavior; use Layout.alignment instead.
qrc:/qml/SignInWaiting.qml:26:9: QML Text: Detected anchors on an item that is managed by a layout. This is undefined behavior; use Layout.alignment instead.
```
- Google login doesn't work. It just doesn't start embedded webbrowser
This also updates the bootstrap tool builder to LLVM 5, but not the ones
we actually use for bootstrap. I'll make that change in a subsequent commit
so as to provide traceable provenance of the bootstrap tools.
A little shim derivation to get this header for Darwin, where it is
needed for cross compilation.
There's no real reason to do glibc and musl like that, but as I'm
maintaining it I suppose I can go overboard like that.