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>