doc: make doc-only -> fallback to user binary

After the #3888 it was not possible to "make doc-only"
in some situations. This now fallsback to any installed
node version and throws "node not found" in error case.

Ref: https://github.com/nodejs/node/pull/3888

PR-URL: https://github.com/nodejs/node/pull/6906


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>