configure.ac:  account for Debian multi-arch
configure.ac:  account for Debian multi-arch

When running `./configure` without `--host` and `--build`, the
`AC_CANONICAL_*` macros run `config.guess` to determine host and build
architectures.

Commit ccf3bc98b from #594 sets `--host` and `--build` to Debian
multi-arch names from variables defined in
`/usr/share/dpkg/architecture.mk` during package builds.

This exemplifies the difference:

    # ./config.guess
    armv7l-unknown-linux-gnueabihf
    # dpkg-architecture -qDEB_BUILD_MULTIARCH
    arm-linux-gnueabihf

This patch is a quick fix for platform detection errors in package
builds, and probably needs something more general in the future.

Fixes #607

kinsamanka/machinekit,RunningLight/machinekit,mhaberler/machinekit,araisrobo/machinekit,RunningLight/machinekit,mhaberler/machinekit,unseenlaser/machinekit,bobvanderlinden/machinekit,unseenlaser/machinekit,mhaberler/machinekit,mhaberler/machinekit,kinsamanka/machinekit,kinsamanka/machinekit,unseenlaser/machinekit,bobvanderlinden/machinekit,RunningLight/machinekit,strahlex/machinekit,ArcEye/MK-Qt5,Cid427/machinekit,mhaberler/machinekit,RunningLight/machinekit,araisrobo/machinekit,Cid427/machinekit,Cid427/machinekit,bobvanderlinden/machinekit,RunningLight/machinekit,unseenlaser/machinekit,ArcEye/MK-Qt5,unseenlaser/machinekit,kinsamanka/machinekit,mhaberler/machinekit,strahlex/machinekit,bobvanderlinden/machinekit,bobvanderlinden/machinekit,strahlex/machinekit,ArcEye/MK-Qt5,Cid427/machinekit,unseenlaser/machinekit,araisrobo/machinekit,araisrobo/machinekit,ArcEye/machinekit-testing,ArcEye/machinekit-testing,ArcEye/machinekit-testing,Cid427/machinekit,araisrobo/machinekit,kinsamanka/machinekit,Cid427/machinekit,ArcEye/machinekit-testing,ArcEye/machinekit-testing,bobvanderlinden/machinekit,strahlex/machinekit,unseenlaser/machinekit,bobvanderlinden/machinekit,Cid427/machinekit,ArcEye/machinekit-testing,strahlex/machinekit,ArcEye/MK-Qt5,unseenlaser/machinekit,mhaberler/machinekit,araisrobo/machinekit,strahlex/machinekit,araisrobo/machinekit,araisrobo/machinekit,RunningLight/machinekit,kinsamanka/machinekit,strahlex/machinekit,kinsamanka/machinekit,kinsamanka/machinekit,mhaberler/machinekit,ArcEye/MK-Qt5,Cid427/machinekit,RunningLight/machinekit,ArcEye/machinekit-testing,araisrobo/machinekit,bobvanderlinden/machinekit,ArcEye/machinekit-testing,ArcEye/MK-Qt5,ArcEye/MK-Qt5,RunningLight/machinekit,ArcEye/MK-Qt5