[fix] #98 fix build and dist clear if folders not exists

Makefile

@@ -34,7 +34,8 @@ format:
 
 # build a wheel package in dist folder
 build:
-	rm -r build/* dist/*
+	if [ -d "./build" ]; then rm -r build/*; fi
+	if [ -d "./dist" ]; then rm -r dist/*; fi
 	python setup.py sdist bdist_wheel
 
 # upload on PyPi repository