Enhance generated export header to more closely match behavior of CMake.
Enhance generated export header to more closely match behavior of CMake.

The CMake script had more logic in it to check compilers and versions,
so I incorporated as much as I could in via predefined macros.

godbyk/OSVR-Core,OSVR/OSVR-Core,leemichaelRazer/OSVR-Core,godbyk/OSVR-Core,OSVR/OSVR-Core,godbyk/OSVR-Core,godbyk/OSVR-Core,leemichaelRazer/OSVR-Core,OSVR/OSVR-Core,godbyk/OSVR-Core,godbyk/OSVR-Core,leemichaelRazer/OSVR-Core,OSVR/OSVR-Core,OSVR/OSVR-Core,leemichaelRazer/OSVR-Core,leemichaelRazer/OSVR-Core,OSVR/OSVR-Core