Two issues are fixed with this commit:
1) When we build OIIO (on unixoid build environments) and no /src/doc/oiiotool was present we had no build target for it (which led to a build error). As we don't need docs for OIIO, we disable it now.
2) We specified a var that OIIO does not recognize (was removed upstream a long time ago): ILMBASE_VERSION.
c647c93f63