buteo-syncfw-qml successfully cross builds a broken package. It installs
the qml plugin below the build architecture multiarch directory. We
track such failures as if the package were failing the cross builds
(FTCBFS). The root cause is that the upstream build system uses the
build architecture qmake executable to query the qml installation
location and therefore gets a build architecture location. debhelper
does pass a host architecture qmake via the QMAKE_EXECUTABLE variable,
so all that needs to be done here is honour that variable. I'm attaching
a patch for your convenience. Please consider forwarding it upstream.

Helmut