Makefile: change the rule to make sure pip is available

Since Python 2.7.9, it's possible to use the "ensurepip" module to
check and bootstrap pip at the same time.  Then, only if that is
not available, we'd try the old bootstrap recipe.

Reference: https://docs.python.org/2/library/ensurepip.htmlSigned-off-by: NCleber Rosa <crosa@redhat.com>

Makefile

@@ -140,7 +140,7 @@ clean:
 	find $(AVOCADO_OPTIONAL_PLUGINS) -name '*.egg-info' -exec rm -r {} +
 
 pip:
-	$(PYTHON) -m pip --version || $(PYTHON) -c "import os; import sys; import urllib; f = urllib.urlretrieve('https://bootstrap.pypa.io/get-pip.py')[0]; os.system('%s %s' % (sys.executable, f))"
+	$(PYTHON) -m pip --version || $(PYTHON) -m ensurepip $(PYTHON_DEVELOP_ARGS) || $(PYTHON) -c "import os; import sys; import urllib; f = urllib.urlretrieve('https://bootstrap.pypa.io/get-pip.py')[0]; os.system('%s %s' % (sys.executable, f))"
 
 requirements: pip
 	- pip install "pip>=6.0.1"