Also, that should be CXX14FLAGS and you may want to add a few things to it, as in
"CXX14FLAGS = -mtune=native -march=native -Wno-unused-variable -Wno-unused-function -Wno-unused-local-typedefs -Wno-ignored-attributes -Wno-deprecated-declarations -Wno-attributes -O3" >> ~/.R/Makevars