Allow all builds to happen with Visual Studio 2015

[kevinhoffman]

WindowsSdk_71A_* and WindowsSdk_81_* environment variables must be set properly in order to build now.

[kevinhoffman]

@diazfran & @rodenweller - forthcoming changes in RA-196 will depend on this. Any concern in merging?

[rodenweller]

What are you trying to accomplish? It really shouldn't need to be this manual. Maybe there's a better way.

[kevinhoffman]

@rodenweller - Different versions of the Windows SDK are required to properly compile the different versions of efsvss to target the different windows versions (some older). This PR does not change that. What this PR does is make the build environment work whether you installed Windows SDK 7.1A first or Windows SDK 8.0 first. It also makes the build consistent with the build setup described here:

https://github.com/ReplibitLLC/ReplibitAgent/blob/develop/Readme.md

So that if you setup a build environment properly for the agent, then you can do an uninterrupted CI build from the agent repo.

[rodenweller]

What I am trying to determine is why the configuration stuff needs to be so manual. VS has excellent built in support for targeting different Windows versions and I'd prefer we use a built in mechanism to accomplish this than a home grown one. We've been able to handle most everything so far by targeting the v140_xp toolset and defining the correct Windows defines. I did a header file in the efs-backup that makes it pretty easy to target multiple versions. It's in root\windows\targetver.h
If you really need to have something really custom, the best way to do that is by modifying or creating a new toolset and setting the Platform Version value. If you want, let me know exactly what you're trying to target and I'll take a crack at it.