Rework the Linux drm kernel module build to leverage off the standard

    kernel build system. This is based on suggestions and examples from
    David Woodhouse. This approach has the advantage that the build
    requirements of a wider range of standard kernels are now supported
    transparently, but the disadvantage of some extra complexity to handle
    building against clean vendor-distributed kernel source trees. This has
    been tested with some recent Red Hat and SuSE distributions.