Alternately use environment variable "VisualStudioVersion" to get version if VC is not installed in default directory

Hi all,
I installed a copy of Visual Studio in "E:\VS2012". This results in line 64 of CMakeLists.txt failing to parse the version from VISUAL_STUDIO_PATH. There's an environment variable, "VisualStudioVersion", which has the version listed as "11.0", so as an alternate method, I get the version # from there if the original code fails.

I think this is pretty conservative - can't see how it would break anything previous, but an alternate would be to just use the environment variable, assuming it's present for other supported versions.

Thx,
--Lee