upower fails to cross build from source, because it fails running the
gtk-doc scanner. Fortunately, the documentation has been separated to an
arch:all -doc package, so running it actually is not necessary during a
cross build, which always is an arch-only build. I'm proposing a patch
to skip running it in arch-only builds in general. Note that demoting
the dependency requires an explicit dependency on docbook-xsl, which was
formerly implied. Once applying the patch, upower can be cross built.

Helmut