djgpp_i586, djgpp_i686: migrate to by-name, migrate lowPrio override to by-name

[guylamar2006]

This pull request restructures and updates the packaging for the DJGPP compiler in Nixpkgs. The main changes include moving and renaming the package for better organization, introducing a new i686 variant, and updating build flags to improve compatibility with older C code and specific dependencies.

Package restructuring and new variant:

• Moved and renamed the DJGPP package from pkgs/development/compilers/djgpp/default.nix to pkgs/by-name/dj/djgpp_i586/package.nix for improved organization.
• Added a new djgpp_i686 package variant, based on djgpp_i586, using the "i686" target architecture.

Build and compatibility improvements:

• Switched from stdenv to gccStdenv to ensure the correct build environment and set stdenv = gccStdenv explicitly.
• Disabled the strictflexarrays1 hardening flag (in addition to format) to prevent build failures with old C code.
• Set CFLAGS = "-std=gnu17" to force the use of the C17 standard for compatibility with GMP 6.2.1's configure script.

Package set cleanup:

• Updated pkgs/top-level/all-packages.nix to reference the new package locations and remove the old DJGPP package definitions.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

[guylamar2006]

nixpkgs-review result

Generated using nixpkgs-review-gha

Command: nixpkgs-review pr 501790
Commit: d90d74118742658916c7146c52668d48b3de7849 (subsequent changes)
Merge: 049830b99b7ccb5d5e6d2c5e2d629106a8596f14

Logs: https://github.com/guylamar2006/nixpkgs-review-gha/actions/runs/23381119029

---

x86_64-linux

✅ 2 packages built:

• djgpp (djgpp_i586)
• djgpp_i686

---

aarch64-linux

✅ 2 packages built:

• djgpp (djgpp_i586)
• djgpp_i686

---

x86_64-darwin (sandbox = relaxed)

✅ 2 packages built:

• djgpp (djgpp_i586)
• djgpp_i686

---

aarch64-darwin (sandbox = relaxed)

✅ 2 packages built:

• djgpp (djgpp_i586)
• djgpp_i686