When trying to install Rcpp_0.12.14 or Rcpp_0.12.9, I get the following error:
../inst/include/Rcpp/Rmath.h:222:55 error: '::Rf_pythag' has not been declared
Does anybody have any idea what the issue is?
I don't know what your local system is but I had exactly the same error for Microsoft R 3.5.0 in Mac OS X 10.13.6, usually, Macs and Microsoft R have some issues either with repositories (it uses snapshot system) or with versions of gcc in general (google for R & Xcode).
I've tried to install Rcpp from console R and it worked perfectly, then I noticed that the version of the fetched package is slightly newer (Rcpp_0.12.17 in console vs Rcpp_0.12.13 in RStudio), so I just made a direct call
install.packages("https://mran.microsoft.com/snapshot/2018-06-01/src/contrib/Rcpp_0.12.17.tar.gz",repos = NULL, type = "source")
and it installed just fine.