SONAME bump for CGAL

[Joachim Reichel]

Hi,

due to some changes in the ABI of ImageIO CGAL bumped its SONAME in 4.6-2. Since
none of the reverse dependencies seems to use that part of CGAL and hasn't
hardcoded "libcgal10" either, a binNMU should do the job.

Unless some maintainer tells me otherwise I plan to ask for binNMUs for your
package once CGAL leaves the NEW queue.

Joachim