Provide a generic autotools helper script (wrapper around build-standalone)

Example usage in CI scripts:

/opt/helpers/build-autotools libsodium https://github.com/jedisct1/libsodium.git src/libsodium 1.0.18

The third argument is an optional branch/tag name to checkout.
It may also be left out in which case 'master' will be assumed by default.