Removes build output from Git tracking

When building on macOS, right after cloning, I got build errors that were solved by running make clean and then building again. The cause is that build output was also tracked in Git, so that some targets that were built on a different machine and CPU architecture were used by the compiler.

This merge request solves this issue by removing all generated files from Git and putting them in .gitignore. A more ideal solution might be to store all build output in a separate folder tree, so not side-by-side with the source code, but that can still be done at a later stage if desired.