Enable python command as optional parameter

If the user writes `make` then `python` is used, but if she
wants to use `python3` then should be able to do it by typing
`PYTHON=python3 make`

[1]: ibayer#105 (comment)

Signed-off-by: Antonio Ossa <[email protected]>

Author: aaossa

Makefile

@@ -1,6 +1,8 @@
+PYTHON ?= python
+
 all:
 	( cd fastFM-core ; $(MAKE) lib )
-	python setup.py build_ext --inplace
+	$(PYTHON) setup.py build_ext --inplace
 
 .PHONY : clean
 clean: