qt: disable webkit on Windows
szip: use _D suffix on Windows

Doesn't work on Visual Studio since this isn't a generator expression,
but ExternalProject doesn't seem to support them fully.
vtk: bump superbuild hash

Contains a fix for Windows (missing includes).
moab: remove username specification
Merge pull request #62 from mathstuf/superbuild-win-fixes

Superbuild fixes for Windows