Add a default makefile target.

src/Makefile

+all: client.so echoserver.so format.so getaddrinfo.so helloworld.so httpserver.so strings.so
+
 %.so: %.pyx
 	python3 setup.py build_ext --inplace $* && mv $*.*.so $@
 
@@ -9,6 +11,6 @@ clean:
 	rm -rf *.cpp *.c
 	rm -rf *.so
 
-.PHONY: clean
+.PHONY: clean all
 .PRECIOUS: %.so
 .SUFFIXES: