Support compilation on macOS using rhub

[eddelbuettel]

The PR contains two short commits which in sum permit the compilation at the macOS machine of rhub. It addressed the puzzle presented in this StackOverflow question.

We change two things:

• do not include omp.h unconditionally as macOS machine may, or may not, have OpenMP
• do not pass an Rcpp::StringVector object to the the Boost tokenizer but take an intermediate step via a std::string

One resulting pass is see here (at least they delete it after a few days). Ignore that I changed the version to 2.0.1.1, that is just a personal habit of making altered tarballs show a different version.

[c0webster]

Thanks a ton, this is super helpful. I'm still a novice to cpp, so I don't think I ever would have figured this out. Cheers.