cmake: do not link against boost in a wholesale (#3)

With the new Beast frontend, RGW now has a small Boost dependency [1] which was
being addressed by statically (and unconditionally) linking *all* the Boost
libraries. This patch ensures that only the necessary Boost components are
linked.

We use the target_link_libraries(<target> <item>...) [2] syntax to ensure that the
library dependencies are transitive: i.e. "when this target is linked into
another target then the libraries linked to this target will appear on the link
line for the other target too."

[1] The boost/asio/spawn.hpp header used by rgw_asio_frontend.cc depends on
    boost::coroutine/boost::context

[2] https://cmake.org/cmake/help/v3.3/command/target_link_libraries.html#libraries-for-both-a-target-and-its-dependents

Signed-off-by: Nathan Cutler <ncutler@suse.com>
Signed-off-by: Kefu Chai <kchai@redhat.com>

src/rgw/CMakeLists.txt

@@ -170,7 +170,7 @@ add_library(radosgw_a STATIC ${radosgw_srcs}
   $<TARGET_OBJECTS:civetweb_common_objs>)
 target_link_libraries(radosgw_a rgw_a ${SSL_LIBRARIES})
 if(WITH_RADOSGW_BEAST_FRONTEND)
-  target_link_libraries(radosgw_a LINK_PRIVATE
+  target_link_libraries(radosgw_a
     ${Boost_COROUTINE_LIBRARY}
     ${Boost_CONTEXT_LIBRARY})
 endif()
@@ -181,7 +181,7 @@ target_link_libraries(radosgw radosgw_a librados
   cls_log_client cls_statelog_client cls_timeindex_client
   cls_version_client cls_replica_log_client cls_user_client
   global ${FCGI_LIBRARY} ${LIB_RESOLV}
-  ${CURL_LIBRARIES} ${Boost_LIBRARIES} ${EXPAT_LIBRARIES} ${BLKID_LIBRARIES}
+  ${CURL_LIBRARIES} ${EXPAT_LIBRARIES} ${BLKID_LIBRARIES}
   ${ALLOC_LIBS})
 # radosgw depends on cls libraries at runtime, but not as link dependencies
 add_dependencies(radosgw cls_rgw cls_lock cls_refcount