SPI - Options to run other compilers (#687)
SPI - Options to run other compilers (#687)

* Change default to use llvm & clang 3.5 rather than 3.4.
* More flexibility for overriding which compiler version.
* Some refactoring to regularize the OIIO and OSL cmake files that overlap.

This doesn't affect non-SPI sites.
aconty/OpenShadingLanguage,imageworks/OpenShadingLanguage,lgritz/OpenShadingLanguage,imageworks/OpenShadingLanguage,lgritz/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,brechtvl/OpenShadingLanguage,lgritz/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,lgritz/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,imageworks/OpenShadingLanguage,brechtvl/OpenShadingLanguage,imageworks/OpenShadingLanguage,aconty/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,aconty/OpenShadingLanguage,imageworks/OpenShadingLanguage,aconty/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,brechtvl/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,gzorin/OpenShadingLanguage,gzorin/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,lgritz/OpenShadingLanguage,gzorin/OpenShadingLanguage,gzorin/OpenShadingLanguage,brechtvl/OpenShadingLanguage,aconty/OpenShadingLanguage,gzorin/OpenShadingLanguage,brechtvl/OpenShadingLanguage