vtkWin32Header: support 64-bit properly on non-MSVC

Change-Id: I8fcd2f1a41884ca083e8b54ce998158594fd095b
vtkWin32Header: always use {Get,Set}WindowLongPtr

These have been in mingw since 2011.

Change-Id: I35cede39029bb18a6fa7dc1d9f27db914e3d763b
Merge topic 'win32-non-msvc-defines' into master

18f64da9 vtkWin32Header: always use {Get,Set}WindowLongPtr
695a8474 vtkWin32Header: support 64-bit properly on non-MSVC