Add make target to distribute executable

That is not working when SDL is not installed.

.gitignore

@@ -38,3 +38,4 @@
 # End of https://www.toptal.com/developers/gitignore/api/c++
 
 bin/
+dist/

makefile

@@ -24,3 +24,7 @@ distclean: clean
 
 exec: $(EXEC)
 	./bin/$(EXEC)
+
+dist: $(EXEC)
+	@mkdir -p dist
+	zip dist/RainbowDodge.zip ./bin/$(EXEC)