how to set compilation options for stan?

[husnoo]

I've got the following:

[nawal@heisenbug:~/data/software-exercises/R]$ cat ~/.R/Makevars 
CXXFLAGS=-O0 -march=native -mtune=native -arch x86_64 -ftemplate-depth-256
CXX11FLAGS=-O0 -march=native -mtune=native -arch x86_64 -ftemplate-depth-256
CXX12FLAGS=-O0 -march=native -mtune=native -arch x86_64 -ftemplate-depth-256
CXX13FLAGS=-O0 -march=native -mtune=native -arch x86_64 -ftemplate-depth-256
CXX14FLAGS=-O0 -march=native -mtune=native -arch x86_64 -ftemplate-depth-256

But when ulam runs, I see the following:

nawal    2095081 68.6  3.5 574808 285348 pts/2   R    16:21   0:02 /nix/store/fg7ass3a5m5pgl26qzfdniicbwbgzccy-gcc-13.2.0/libexec/gcc/x86_64-unknown-linux-gnu/13.2.0/cc1plus -quiet -I /nix/store/4kbfmgdfg0h6zjd0kjayhsm314s3bmjn-R-4.3.3/lib/R/include -I /nix/store/6cn0x7wci7wa6vhhz07pvh7ziqaqimlb-r-Rcpp-1.0.12/library/Rcpp/include/ -I /nix/store/qfh1qgk5h4x34cfingbgl83wqrrd3fl8-r-RcppEigen-0.3.4.0.0/library/RcppEigen/include/ -I /nix/store/qfh1qgk5h4x34cfingbgl83wqrrd3fl8-r-RcppEigen-0.3.4.0.0/library/RcppEigen/include/unsupported -I /nix/store/ix342nzw09l2iqyss22scqq9fl4v1dy6-r-BH-1.84.0-0/library/BH/include -I /nix/store/hyiz44s2rciwdid2h847acw82z294ccd-r-StanHeaders-2.32.6/library/StanHeaders/include/src/ -I /nix/store/hyiz44s2rciwdid2h847acw82z294ccd-r-StanHeaders-2.32.6/library/StanHeaders/include/ -I /nix/store/dhlx17iaf7rark4lc7m3i2hvasz86191-r-RcppParallel-5.1.7/library/RcppParallel/include/ -I /nix/store/5n9f14hwzlwbrsw0jij876zba42277k5-r-rstan-2.32.6/library/rstan/include -D_GNU_SOURCE -D NDEBUG -D EIGEN_NO_DEBUG -D BOOST_DISABLE_ASSERTS -D BOOST_PENDING_INTEGER_LOG2_HPP -D STAN_THREADS -D USE_STANC3 -D STRICT_R_HEADERS -D BOOST_PHOENIX_NO_VARIADIC_EXPRESSION -D _HAS_AUTO_PTR_ETC=0 -D _REENTRANT -D RCPP_PARALLEL_USE_TBB=1 -include /nix/store/hyiz44s2rciwdid2h847acw82z294ccd-r-StanHeaders-2.32.6/library/StanHeaders/include/stan/math/prim/fun/Eigen.hpp -idirafter /nix/store/rzs2mn1lv3dsiyilygfz9vv1p8r36jvn-glibc-2.39-52-dev/include -idirafter /nix/store/fg7ass3a5m5pgl26qzfdniicbwbgzccy-gcc-13.2.0/lib/gcc/x86_64-unknown-linux-gnu/13.2.0/include-fixed -idirafter /usr/include file1ff5872073811a.cpp -quiet -dumpbase file1ff5872073811a.cpp -dumpbase-ext .cpp -mtune=generic -march=x86-64 -g -O2 -std=gnu++17 -fpic -o /tmp/cc3A9Lgb.s

i.e. -O2 instead of -O0. Is this something set in rethinking? Or does it mean R can't find the ~/.R/Makevars for some reason?