Refactor ANTLR3 dependency (#6202)

Refactor ANTLR3 dependency (#6202)

This PR refactors our first, and arguably most fragile, dependency.
Right now all dependencies need to be manually installed (by calling the appropriate contrib/get-X script). For optional dependencies, we additionally need to enable them when calling the configure script (or via ccmake).
This PR is the first step in refactoring all dependencies to be automatically build (if required) as an external project.

Note that this not only eliminates the need to call contrib scripts, but also simplifies cross compilation: as all dependencies are now built within the build folders, every build folder has its own copy which may use different toolchains.