I began playing this afternoon with building Processing using Travis CI. Travis is a continuous integration platform for open source projects. That means it attempts to compile and run tests for Processing on every pull request and commit, and can let you know when the build failed.

For the moment it does not run any tests (I couldn't figure out how to run them) but it attempts to build everything so it should catch the most flagrant errors. This could be improved later so that it makes sure that commits adhere to a certain code style, files are in the correct encoding and no extraneous whitespace is added.

This is how a build looks: https://travis-ci.org/federicobond/processing. Of course, you don't even have to visit their website. Travis can notify GitHub directly so that you see the status of the build right besides the merge button.

If you are interested in this, I can submit a PR.