debian: Link against libfreebsd-glue on kfreebsd

cpuset_getaffinity() is available on kfreebsd but not in libc but in
libfreebsd-glue. This code adds the library if kfreebsd is detected so
cpuset_getaffinity() can be used.

Without this, tuklib_cpucores will detect `cpuset' because the compile
test succeeds and later it fails at link phase.

[ imported from uploaded NMU]