mshr build fails against external CGAL 4.13 - Mesh_3::internal vs internal::Mesh_3

[Stefan Brüns]

With CGAL 4.13, "Uniformized between Mesh_3::internal and internal::Mesh_3"
(https://github.com/CGAL/cgal/commit/1078e00e7f46655a)) the wrapping
order has been swapped.

The attached patch fixes the problem.

[Drew Parsons]

Merged in dparsons/cgal5.6 (pull request #17)

build against CGAL 5 and CGAL 5.6

Closes: #98, #101.

→ <<cset a44b0f93f268>>