Rcpp Makevars: portable -mpopcnt flag

I am using Rcpp and I am trying to make my Makevars file written to be portable. Initially this is my file:

PKG_CXXFLAGS = -std=c++11 -mpopcnt
PKG_CXX1XFLAGS = $(PKG_CXXFLAGS)

Then I modified it so I can make the C++11 flag portable:

CXX_STD = CXX11
PKG_CXXFLAGS = -mpopcnt
PKG_CXX1XFLAGS = $(PKG_CXXFLAGS)

However, -mpopcnt is not a portable flag. Is there a fix for this?