build: Fix bug with enabling use of system-installed boost libraries

When the flag is specified, the script did not properly set
``want_system_boost`` variable.

Also, ndnboost/ doesn't have enable_shared_from_this header...

refs #1126 (http://redmine.named-data.net/issues/1126)

Change-Id: I6c68a298f5343762e3631f5731aa3252dcf51f6b