[RFC] Add dependency rebuilding (32/64 bit).

This has been used to generate neovim/deps#4. Here's a full Travis test build: https://travis-ci.org/fwalch/bot-ci/builds/35124444. Output is in https://github.com/fwalch/deps/tree/testing.
A couple ideas for another PR: `require_...` that takes variadic args like this:

    require_environment_variable "${BASH_SOURCE[0]}" ${LINENO} ${subtree} ${dir} ${repo} ${branch}

Or, make `require_` return the value of the var if the var exists so we can initialize at the same time:

    local branch=require_env_var "${prefix}_BRANCH" "${BASH_SOURCE[0]}"

(Although LINENO is cool, I don't think it is worth the verbosity for our "little" collection of scripts.)