j4-dmenu-desktop: 2.18 -> unstable-2023-09-12, clean up

[TomaSajt]

Description of changes

Closes #281272
Closes #280925

This PR fixes the build for the package build by building from the currently latest commit.
It also has some cleanup:

Changes:

• use finalAttrs instead of rec
• use hash instead of sha256
• add meta.changelog
• add meta.mainProgram
• meta.description: "recognize" -> "recognizes"
• meta.license: gpl3 -> gpl3Only (not sure if gpl3Plus is acceptable, so I went with gpl3Only)

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.

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[cole-h]

It looks like this package hasn't had a release in over 3 years now -- would you mind updating it to the latest commit (and change its version to unstable-)? It appears the compilation issue has been fixed upstream, as mentioned in this comment on a similar PR (#280925 (comment)), so we shouldn't need to set CXXFLAGS anymore after that.

[cole-h]

Thanks! Appears to be working fine for me.