Build: allow local static png dependency

To start with, I am very new to building with windows SDK - so this issue is probably related with a bad practise: please tell.

Since commit 386b4e53bb I have been unable to build using a local static png dependency (the one generously provided online by: http://www.lfd.uci.edu/~gohlke/pythonlibs/#matplotlib). Previously it would build even with png version = None ; now a `CheckFailed` error is raised.

As a workaround I ended adding the same try / catch pattern in check method of
`Png` class as in the `LibAgg` class of `setupext.py`. I am really unsure this the good thing to do ; however in this case I would be happy to learn.