This commit adds the build-tools repo to the mock cfg on el5.

Since Ruby 1.9 requires a newer autoconf, we're adding a build of our m4
and autoconf to a build-tools repository. This repository will be hosted
on the distribution server. Items in PE should *never* require anything
in this repo for runtime, only for builds.

Signed-off-by: Michael Stahnke <stahnma@puppetlabs.com>