fix win32 build

I had a few minor issues building node-canvas on win32 following your guide at https://github.com/LearnBoost/node-canvas/wiki/Installation---Windows
* PNG.h includes cairo.h which resides in GTK/include/cairo
* undefined references to libpng symbols
* unknown CAIRO_FORMAT_RGB30 for the outdated (but at least precompiled) gtk+bundle you referenced

These changes worked for me.