Add a .gitignore file.

Make git ignore files that should not be version-controlled, such as object
files.
Tweak the makefile

Make the compiler emit warnings. The compiler can find many potential issues
before they become nasty bugs, so let's take advantage of that capability.

Remove `sfml-app` from the list of phony targets. Categorizing the executable
file `sfml-app` as a "phony" file is simply incorrect: the executable is a real
file.