bootstrap: detect tools required by gnulib-tool

List tools needed by gnulib-tool, whether or not the project
also requires the tools in cfg.mk $buildreq.  In particular,
detect if the project uses gnulib-local diffs, in which case
patch is a prerequisite.

* build-aux/bootstrap (buildreq): Provide minimum implicit
dependencies.
* DEPENDENCIES: Mention patch as a prereq.

Signed-off-by: Eric Blake <eblake@redhat.com>