Specify exact eclipse version required to build

[mdaniel]

One will observe that the Building wiki just says "download Eclipse" but there never has been just one Eclipse. Even identifying the version by its codename is often not specific enough.

Also, perhaps tangentially, there is no version qualifier on the org.eclipse.core.runtime (or any of its friends) which has to bad outcomes: one cannot guess the Eclipse distribution to download, and it prevents Eclipse from even knowing what version of the API you support (because it's clearly not all of them).