The current version of purify needs a patched version of the eigen3
package. To avoid shipping a whole copy of eigen3 with the sources, the
orginal header files are copied to the sources during dh_auto_config,
and then the patch `debian/eigen3_LukePratley.patch` is applied.

It is expected that the patch will merge with the upstream eigen3 at some
point.

Ole Streicher <olebole@debian.org>
