Fix cmake for linux compilation.

[Conzar]

Before this patch, compilation would fail with the following error

error: #error This file requires compiler and library support for the ISO C++ 2011 standard. This support must be enabled with the -std=c++11 or -std=gnu++11 compiler options.
 #error This file requires compiler and library support 

This PR fixes the above error and allows OSVR-fusion to be compiled on linux.

[nanospork]

Thanks for this. Changes look good, but I'd like to confirm that it doesn't interfere with building in VS on Windows before I merge. Will be another week or so.

[godbyk]

Should work fine on Windows. I do the same thing in the SteamVR-OSVR CMakeLists.txt file.

[Conzar]

Have you tried this yet?