Add support for Clang 6.0.0rc2 in SPI makefiles
Add support for Clang 6.0.0rc2 in SPI makefiles

aconty/OpenShadingLanguage,aconty/OpenShadingLanguage,lgritz/OpenShadingLanguage,imageworks/OpenShadingLanguage,lgritz/OpenShadingLanguage,aconty/OpenShadingLanguage,brechtvl/OpenShadingLanguage,brechtvl/OpenShadingLanguage,lgritz/OpenShadingLanguage,brechtvl/OpenShadingLanguage,imageworks/OpenShadingLanguage,lgritz/OpenShadingLanguage,imageworks/OpenShadingLanguage,aconty/OpenShadingLanguage,imageworks/OpenShadingLanguage,imageworks/OpenShadingLanguage,brechtvl/OpenShadingLanguage,aconty/OpenShadingLanguage,brechtvl/OpenShadingLanguage,lgritz/OpenShadingLanguage