Armadillo 7.900.0 and C++11 transition about rcpparmadillo HOT 4 CLOSED

1. This was primarily done in src/Makevars with the 7.899.0 branch. However, the Rcpp.plugin.maker() option needs to be updated per #127.
2. Probably should have CXX_STD = "CXX11" option in Rcpp.package.skeleton()?
3. Use autoconf AX_CXX_COMPILE_STDCXX_11 macro, then set CXX_STD to either CXX98 or CXX11.

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Instead of 3, which R already does, just ask R?

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Let's discuss later. I think I have most of these licked by outsourcing to what R already has -- see Writing R Extensions on OpenMP.

I also want users to opt-into C++11 and not enforce it.

And I just commented in #127 too.

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I think we have this under control now: C++11 is an option for package users of RcppArmadillo.

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