The Makevars
CXX_STD = CXX14
CXX14 = g++ #-7.3.0 # or clang++ if you have that
CXX14FLAGS = -O3 -fPIC -Wno-unused-variable -Wno-unused-function -DBOOST_PHOENIX_NO_VARIADIC_EXPRESSION
Did the job. Is unfortunate that RStan 2.18 is forced to be hard to install for C++14 adoption