configure: don't check the presence of D3D11/D3D9 structures
Either D3D11/D3D9 is usable and the structures are there, or they are not. We don't support old mingw-w64 versions without these headers.
- D3D11 is not enabled if libavcodec/d3d11.h is not usable
- DXVA2 is not enabled if libavcodec/dxva2.h is not usable
- direct3d9_filters is enabled if d3d9.h is usable (ie not UWP builds)