cmake: fix build WITH_SYSTEM_BOOST=ON

cmake: fix build WITH_SYSTEM_BOOST=ON

FindBoost.cmake from upstream cmake now finds python libraries like

find_package(Boost 1.67 python36)

and it export targets like Boost::python36

but we are still linking against Boost::python, so to be compatible
with FindBoost.cmake, we need to update BuildBoost.cmake and
mgr/CMakeLists.txt accordingly. in other words, to export
Boost::python36 and to link Boost::python36.

Signed-off-by: Kefu Chai <kchai@redhat.com>