slic3r: new package

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

NOTE 1: 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.

NOTE 2: The tests fail, so we skip them. And 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.