If a system is multiarch-enabled (contains libs for more than one arch), the Qt buildsystem will pick the first arch available instead of the one currently used. For example if in an amd64 system one has the libs installed for arm and amd64, with the headers only for amd64, and one performs a native build, the build system will choose the first available arch, which will be arm in this case.

A more detailed explanation can be found at <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=683036>