Make building the test application optional

[adrianbroher]

When integrating the gl3w project into bigger projects it would be preferable to disable every optional code to reduce build time a bit and to remove clutter from the project dir.

[skaslev]

Currently, SConstruct file's sole purpose is to build the test applications.

If you want to incorporate gl3w into your application I would suggest running gl3w_gen.py once and then using the generated files in your project with the build system of your choice.

With that said, gl3w is open source and any code contributions are welcome.