CMake fixes for latest ILMBase lib (2.1)

In Blender project, we have an helper bash script users can use to automatically install and/or compile needed dependencies, and we use CMake tool to build needed libs.

This patch fixes OSL cmake for linux with latest openEXR/ilmbase lib names.

I also had to change a friend declaration, else ti did not worked here (gcc4.8)…

Thanks for checking it,
Bastien (aka mont29).