Microsoft has stipulated, that in the future,
all DirectX applications will need to use a DLL named D3DCOMPILER_47, if not of some greater version. What this driver does seems quite useful.
In the past, it had always been a limitation in the design of raster-based graphics, that actual shader code needed to be fed to the graphics drivers directly as text. As an alternative, D3DCOMPILER allows for the shader code to be compiled into bytecode, which DirectX drivers can understand. This also has uses in obfuscating copyrighted content. But mainly, it provides performance improvements in fact, because many games today have highly complex “Compute Shaders” etc..
When we install the Windows SDK on a Windows 7 or a Windows 10 computer, not the same libraries and header files will install each time.
(Some Text Deleted Because Erroneous Before.)
(Edit 09/18/2016 : )
I have learned, that the library D3DCOMPILER_47.DLL is supposed to reside in both the System32 and the SysWOW64 folders (on a 64-bit Windows system), and is auto-generated there, much as D3DX9_43.DLL was, by the DirectX system on the computer, which is supposed to be running the game in question.
Because the DirectX system on Windows 7 will generate at maximum, D3DCOMPILER_43.DLL , some game developers have been including their own version of the _47 DLL, in the same directory as the EXE File, which is also the first directory the EXE File tends to search for it.
But a problem with that is, that some version of this library will be built specifically for Windows 8.1 for example, and will therefore not run properly on Windows 7. This produces an obscure error message, which I do not recall the exact spelling of.
Yet, the onus is not on the OGRE developers to supply their own version of this library. They do supply a version with their SDKs specifically, but that one is now built to run on Windows 8.1 as well.
Further, version _47 may only be needed for DirectX 11 applications, while _43 may still be good enough for Legacy DirectX 9. Yet, if OGRE was coded to use _47, then it will use that version throughout, even if I was only to try building their DirectX 9 rendering system.
The correct Path on my Windows 7 box ‘Mithral’ for this DLL, is
C:\Program Files (x86)\Windows Kits\8.1\Redist\D3D\x86
Contrarily to what the spelling of this Path may suggest, the DLL version I find there does load correctly on Windows 7. And, I am based on the conventional 32-bit build of OGRE. The above Path is related to one, which will also reveal the 64-bit DLL.
As for OGRE still requiring the Legacy DirectX SDK to build, this is a question of which version of the header files it has been programmed to expect, as well as of providing both the DXGUID and D3DX Static Libraries, whose file-names end in .LIB, as opposed to any DLL Files, which are the Dynamically Linked Libraries.
It is common, for the compilation of software to rely on such static libraries, whereas to run the software should only require the DLL Files, under Windows.