handling C++14 in sourceCpp

[romainfrancois]

sourceCpp does not handle C++14. Pros because of this:

  if (is.mavericks.system() && is.snowleopard.R()) {
    ## work around USE_CXX1X breaking on this combo
    buildEnv[["PKG_CXXFLAGS"]] <- paste(buildEnv[["PKG_CXXFLAGS"]], "-std=c++11")
  } else {
    buildEnv[["USE_CXX1X"]] <- "yes"
  }

We need some way to go from Rcpp = "Rcpp14" to setting up C++14 parameters.