slic3r: new package

Slic3r is a G-code generator for 3D printers.

- Math-Clipper and Boost-Geometry-Utils have been bumped to satisfy
  Slic3r.

- Slic3r has problems with perl 5.16 due to a locale issue (comma vs
  period in floating point numbers). So we use perl 5.14.

- The tests fail, so we skip them. According to the author of Slic3r,
  that should be safe:

    "Tests failed because of a typo when the 0.9.10b tag was applied.
    You can safely ignore the test results, Slic3r will work."

  See https://github.com/alexrj/Slic3r/issues/1303

  For reference, the errors look like this:

    Use of uninitialized value $deg in numeric eq (==) at /tmp/nix-build-perl-slic3r-0.9.10b.drv-0/git-export/t/../lib/Slic3r/TriangleMesh.pm line 328.
    # Looks like your test exited with 255 before it could output anything.