libgtkdatabox fails to cross build from source, because the gtk-doc
scanner fails to run as it is a host executable. Fortunately, the
documentation has been separated to an Arch:all package, so we don't
actually have to run it. I propose disabling gtk-doc in arch-only builds
and doing so fixes the cross build without inducing changes to binary
artifacts of native builds. I'm attaching a patch for your convenience.

Helmut