Qt's configure grabs the path to xkb's data root during configure, but the
build changes in 5.8 apparently broke the handling for cross builds. As a
result, the string embedded in the binary depends on whether or not some files
are present in the builder's filesystem.

The "-xkb-config-root" configure setting is intended to allow manual overriding
but it is also broken. See: https://bugreports.qt.io/browse/QTBUG-60005

This has since been fixed upstream, so just hard-code the path for now. We can
drop this patch when we bump to a fixed Qt.

Also, fix the "-qt-xkbcommon-x11" config param which was renamed. This does not
appear to affect build results, presumably because auto-detection is working,
but it does not hurt to be explicit.

This is a backport of Core's PR14000

Depends on D1822