Add C++14 for RcppArmadillo v15 compatibility

[ycl6]

A quick patch so that the package can be compiled when using RcppArmadillo v15.

Starting from release 15.0.0, the minimum compilation standard is C++14.
https://cran.r-project.org/package=RcppArmadillo

compiler_check.hpp:82:4: error: #error "*** C++14 compiler required; enable C++14 mode in your compiler, or use an earlier version of Armadillo"
   82 |   #error "*** C++14 compiler required; enable C++14 mode in your compiler, or use an earlier version of Armadillo"
      |    ^~~~~