Allow using wxGraphicsContext with MinGW if gdiplus.h is available.

Allow using wxGraphicsContext with MinGW if gdiplus.h is available.

MinGW doesn't include gdiplus.h but it can be installed independently by the
user. Allow to use it if it's available:

- Include windows.h when checking for it in configure as this is apparently
  needed for its compilation.
- Don't reset wxUSE_GRAPHICS_CONTEXT to 0 for non-Microsoft compilers, instead
  just define it as 0 by default for them, allowing the user to simply change
  the definition in wx/msw/setup.h.

Closes #11892.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63832 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775