mupdf: Improve pkg-config file

[OPNA2608]

Description of changes

• Use variable substitution to base paths on prefix
  • multiple-outputs hook takes care of applying corrections as needed
• Actually use the variables in Cflags & Libs

---

Gives:

prefix=/nix/store/qs7s1r5z3k2nb7c4c31k3k4zs7bz6h4h-mupdf-1.23.5
libdir=${prefix}/lib
includedir=/nix/store/9k7v3h2ykh92bahprg04y1cngpri7dpv-mupdf-1.23.5-dev/include

Name: mupdf
Description: Library for rendering PDF documents
Version: 1.23.5
Libs: -L${libdir} -lmupdf
Cflags: -I${includedir}

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

---

Add a 👍 reaction to pull requests you find important.

[lilyinstarlight]

Diff applied to base branch, thank you!