message: use pathsubst for CMDSTAN_MAIN_O
diff: # Models (to be passed through stanc)
  ##
  CMDSTAN_MAIN ?= src/cmdstan/main.cpp
  -CMDSTAN_MAIN_O ?= src/cmdstan/main.o
  +CMDSTAN_MAIN_O = $(patsubst %.cpp,%.o,$(CMDSTAN_MAIN))
  STAN_TARGETS = $(patsubst %.stan,%$(EXE),$(wildcard $(patsubst %$(EXE),%.stan,$(MAKECMDGOALS))))
  