Add option for WinRT.

"if( ${CMAKE_SYSTEM_VERSION} EQUAL 6.2 )" works for all versions of
Win8.
These changes will suppress compile error when building except for RT.
Merge pull request #207 from SubjectName/Compile_Option_fix

Add option for WinRT.
Renamed CMake -DFORRT option to -DWINRT
Merge pull request #1 from RobertBeckebans/master

Sync from original