make distclean doesn't remove everything

[wohali]

Running a make distclean in a checkout of Apache CouchDB from git should remove all dependencies in other repos, including:

• config
• b64url
• ets_lru
• khash
• snappy
• docs
• fauxton
• folsom
• hyper
• ibrowse
• jiffy
• mochiweb
• meck
• recon
• bear

It should also remove the rel/couchdb release folder, and any apache-couchdb-... dist tarball/folder.

In short, after running make distclean, the directory should look exactly like it would from a fresh clone.

In working on this, it would be acceptable for the distclean Makefile to shell out to commands like git reset --hard and git clean -ffdx but that's not sufficient to remove the external repo dependencies, since they're in .gitignore.