Error in cmake if optix version > 5.1.0

When one uses optix 5.1.1, an error is generated due the install directory being hardcoded to 5.1.0. It is even worse when using optix 6.0.0, as the name of the dlls does not follow the same syntax as the one from versions 5.1.x (also hardcoded...). Thus the user must manually set all the lib and dll in the cmake cache.

This branch should solve these problems by checking which version is installed.

Note that for the moment this will only work with Windows, and if Optix has been installed to its defaults install directory. For linux, this branch should note introduce any changes.