Motivation for this change

Updates k2pdfopt to 2.42 for #28643

I use this package infrequently, but I'd like to see it working in 17.09. It was a pain to package properly--it includes a number of .c and .h files that are intended to replace the corresponding files in the upstream packages. I've used overrideAttrs to enable this more nicely than including an ad-hoc build script like before. (Note that I had to use mupdf 1.10a instead of the more recent version in nixpkgs 1.11, since the .c files in k2pdfopt are written just for 1.10a)

This also includes a modification to "gocr" to install libraries and include files so it can compile in k2pdfopt. However, I've disabled the GOCR engine since it crashes for me while testing. The tesseract OCR engine works much better in most cases anyway.

I've tested the executable works, as well as the optional functionality enabled by tesseract, mupdf, ghostscript, and jasper.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.