Fix compile error with Visual C++

Microsoft's Visual C++ doesn't support C99 and so variable length arrays, `isnan()`, and `isinf()` don't compile. These commits address that.

The discussion for these changes [took place here](https://github.com/LearnBoost/node-canvas/pull/373/files#diff-d52111b9458a13e196dab8299f822c29R2035).

This should fix Issue #420.