Make building of OSL plugins optional
Make building of OSL plugins optional

Those aren't always needed and might give some extra pain to setup
building static libraries on Linux due to the -fPIC crazyness.

amartinez-cg/OpenShadingLanguage,gzorin/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,svenstaro/OpenShadingLanguage,lgritz/OpenShadingLanguage,brechtvl/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,brechtvl/OpenShadingLanguage,gzorin/OpenShadingLanguage,lgritz/OpenShadingLanguage,lgritz/OpenShadingLanguage,brechtvl/OpenShadingLanguage,aconty/OpenShadingLanguage,svenstaro/OpenShadingLanguage,imageworks/OpenShadingLanguage,imageworks/OpenShadingLanguage,gzorin/OpenShadingLanguage,svenstaro/OpenShadingLanguage,imageworks/OpenShadingLanguage,gzorin/OpenShadingLanguage,lgritz/OpenShadingLanguage,imageworks/OpenShadingLanguage,aconty/OpenShadingLanguage,brechtvl/OpenShadingLanguage,lgritz/OpenShadingLanguage,imageworks/OpenShadingLanguage,aconty/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,brechtvl/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,aconty/OpenShadingLanguage,aconty/OpenShadingLanguage,svenstaro/OpenShadingLanguage,gzorin/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,fpsunflower/OpenShadingLanguage