This GUID changes depending on what version of the DirectX SDK you have.
The GUID is defined in d3d11shader.h as a const global, not a preprocessor
macro, so this #ifndef was clobbering it always. This caused DX11 shader
compliation to fail when using the Windows 8.1 SDK.
The D3D11 shader compiler doesn't remove unused constants because it would change the size of buffers uploaded by the application. This means that all of the default unused bgfx uniforms take up a huge amount of space that forces the app to waste a ton of time uploading zeroes. This change is a bit of a hack but removes the unused constants and improves performance a great deal.
