gcd : Added unit test and Makefile target

gcd/Makefile

@@ -11,6 +11,10 @@ bin/gcd: src/gcd.cpp | check-cxx-works bin
 	@echo Compiling $<
 	@${CXX} ${CFLAGS} $^ -o $@
 
+bin/gcd_test: src/gcd_test.cpp | check-cxx-works bin
+	@echo Compiling $^
+	@${CXX} ${CFLAGS} $^ -o $@
+
 .PHONY: install
 install: bin/gcd
 	@cp -v $^ /usr/bin
@@ -19,6 +23,10 @@ install: bin/gcd
 uninstall: /usr/bin/gcd
 	@rm -v $^
 
+.PHONY: test
+test: bin/gcd_test
+	@./$^
+
 .PHONY: clean
 clean:
 	@rm -rfv bin