message: specify else cases for compile flags
diff: @@ -97,15 +97,23 @@ endif
  ifdef STAN_THREADS
  STAN_FLAG_THREADS=_threads
  +else
  +STAN_FLAG_THREADS=
  endif
  ifdef STAN_MPI
  STAN_FLAG_MPI=_mpi
  +else
  +STAN_FLAG_MPI=
  endif
  ifdef STAN_OPENCL
  STAN_FLAG_OPENCL=_opencl
  +else
  +STAN_FLAG_OPENCL=
  endif
  ifdef STAN_NO_RANGE_CHECKS
  STAN_FLAG_NO_RANGE_CHECKS=_nochecks
  +else
  +STAN_FLAG_NO_RANGE_CHECKS=
  endif
  STAN_FLAGS=$(STAN_FLAG_THREADS)$(STAN_FLAG_MPI)$(STAN_FLAG_OPENCL)$(STAN_FLAG_NO_RANGE_CHECKS)
  