[MRG] More on the MacOS build

I'm still struggling with this; meanwhile it seems that `master` has lagged behind. 

This contains 2 changes: 
-  build _inference.so with pyLpInference.cxx on MacOS too (why discriminate? ;)
-  Instead of defining `uint` as `unsigned int` with compiler flags, import the appropriate (I hope) numpy header file in `nifty_iterator`. This way, in case on some system or on some version of numpy the typedef is different, this will ensure consistency (as I assume everywhere that nifty_iterator was used, the data passed to it was declared using the numpy version of the typedef, but I may be wrong!)

The second change is not conditional to MacOS X and would affect the codebase on any platform (!) but in my opinion it was a bug.  An alternative fix would be to directly use `unsigned int` instead of `uint` if this is what's needed there; I didn't grep the code to see how it's called.

I have some test failures and the Python wrapper still segfaults but I'll open a linked issue for that.