Add PYTHON variable for python2/python3 make install

Makefile

@@ -2,7 +2,8 @@ MININET = mininet/*.py
 TEST = mininet/test/*.py
 EXAMPLES = mininet/examples/*.py
 MN = bin/mn
-PYMN = python -B bin/mn
+PYTHON ?= python
+PYMN = $(PYTHON) -B bin/mn
 BIN = $(MN)
 PYSRC = $(MININET) $(TEST) $(EXAMPLES) $(BIN)
 MNEXEC = mnexec
@@ -54,13 +55,13 @@ install-manpages: $(MANPAGES)
 	install -D -t $(MANDIR) $(MANPAGES)
 
 install: install-mnexec install-manpages
-	python setup.py install
+	$(PYTHON) setup.py install
 
 develop: $(MNEXEC) $(MANPAGES)
 # 	Perhaps we should link these as well
 	install $(MNEXEC) $(BINDIR)
 	install $(MANPAGES) $(MANDIR)
-	python setup.py develop
+	$(PYTHON) setup.py develop
 
 man: $(MANPAGES)