make-dist: exclude unused bits in boost

make-dist: exclude unused bits in boost

the docs, examples and tests are not used. so drop them. we could go
further by removing unused components in boost. but that'd be an issue
if somebody added a component in CMakeLists but forgets to update this
script. also, we need to remove boost/$component and lib/$component to
achieve this goal. this also introduces extra complicity. so leave it
for another change.

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