[CMake] VC2015 and Windows 7 compatibility

[Gonzalo Garramuño]

I compile on Windows 8.1.  I just found out that my program does not 
work on Windows 7 after I upgraded my toolchain to VC2015. Users of my 
program on Windows 7 get:

Missing DLL ms-api-win-core-libraryloader-l1-2-0.dll

After some digging, I found out that it can be because the use of SDK 
Sets on win8.1.  And that perhaps setting the compiler compatibility to 
WinXP should fix it.  However, I don't know how to do that from cmake.

Any ideas?

If you want to see my CMakeLists.txt, you can get the code with:

$ git clone ssh://ggarra13@git.code.sf.net/p/mrviewer/code mrviewer-code


-- 
Gonzalo Garramuño