Add test/common/poller to .gitignore
Document a bug that I've just found when testing this pull request

Basically, it is bad practice to rely on global objects, because their
constructor is only called when they are embedded into a dyamic lib.

We should fix this problem, and the proper way to do that is IMHO to use
the singleton pattern, however fixing the problem in this pull request is
out of scope, so I'm just dropping there a comment.