message: Add rule for cleaning up compiled stan programs
  Can use via:
  make STANPROG="examples/bernoulli/bernoulli" clean-program
  It is also safe to specify the stan file instead:
  make STANPROG="examples/bernoulli/bernoulli.stan" clean-program
  In this example bernoulli, bernoulli.d, bernoulli.o, and
  bernoulli.hpp would be removed.
diff: @@ -205,7 +205,7 @@ endif
  ##
  # Clean up.
  ##
  -.PHONY: clean clean-deps clean-manual clean-all
  +.PHONY: clean clean-deps clean-manual clean-all clean-program
  clean: clean-manual
  $(RM) -r test
  @@ -227,6 +227,15 @@ clean-all: clean clean-deps clean-libraries clean-manual
  $(RM) -r $(CMDSTAN_MAIN_O)
  $(RM) $(wildcard $(STAN)src/stan/model/model_header.hpp.gch)
  +clean-program:
  +ifndef STANPROG
  + $(error STANPROG not set)
  +endif
  + $(RM) $(wildcard $(patsubst %.stan,%.d,$(basename ${STANPROG}).stan))
  + $(RM) $(wildcard $(patsubst %.stan,%.hpp,$(basename ${STANPROG}).stan))
  + $(RM) $(wildcard $(patsubst %.stan,%.o,$(basename ${STANPROG}).stan))
  + $(RM) $(wildcard $(patsubst %.stan,%$(EXE),$(basename ${STANPROG}).stan))
  +
  ##
  # Submodule related tasks
  ##
  