Update makefile

makefile

-.PHONY: clean pythran
+.PHONY: clean
 
-all: C pythran
+all: helloworld
 
-C:
+helloworld:
 	  gcc helloworld.c -o helloworld.e
 
-pythran:
-	  pythran rosenbrock.py -o rosenbrock_pythran.so
-
 clean:
 	  rm -rf *.o *.e *.so *.pyf *.pyc __pycache__