feat: build ROOT against external llvm

[wdconinc]

Briefly, what does this PR introduce? Please link to any relevant presentations or discussions.

ROOT includes its version of llvm, but it should be possible to build against an external llvm. With ROOT 6.36, that external llvm is at the same level as what ROOT requires. This PR enables the external llvm build.

Needs:

• root: fix cling CMake include ordering for external LLVM eic-spack#932
• chore: root-6.40.02 #298

What is the urgency of this PR?

• High (please describe reason below)
• Medium
• Low

What kind of change does this PR introduce?

• Bug fix (issue #__)
• New feature (issue: ROOT with external llvm)
• Optimization (issue #__)
• Updated documentation
• other: __

Please check if any of the following apply

• This PR introduces breaking changes. Please describe changes users need to make below.
• This PR changes default behavior. Please describe changes below.
• AI was used in preparing this PR. Please describe usage below.

[Copilot]

Pull request overview

Enables building ROOT against an external LLVM (instead of ROOT's bundled copy) by adding ~builtin_llvm to the ROOT require list, and pulls in the corresponding spack-packages cherry-pick that introduces the builtin_llvm variant.

Changes:

• Add cherry-pick 8cd9959e to spack-packages.sh that adds the builtin_llvm variant to the ROOT package recipe.
• Add ~builtin_llvm to the ROOT requirements in spack-environment/packages.yaml to use the external LLVM.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
spack-packages.sh	Adds new spack-packages cherry-pick hash and matching descriptive comment for the ROOT builtin_llvm variant.
spack-environment/packages.yaml	Adds ~builtin_llvm to ROOT's required variants, switching to external LLVM.

[github-actions]

Capybara summary for PR 288

• rec_dis_10x100_minQ2=1000_epic_craterlake (new artifact) (ref artifact)
• rec_dis_10x100_minQ2=1_epic_craterlake (new artifact) (ref artifact)
• rec_dis_18x275_minQ2=1000_epic_craterlake (new artifact) (ref artifact)
• rec_dis_18x275_minQ2=1_epic_craterlake (new artifact) (ref artifact)
• rec_dis_5x41_minQ2=1_epic_craterlake (new artifact) (ref artifact)
• rec_e_1GeV_20GeV_epic_craterlake (new artifact) (ref artifact)
• rec_pi_1GeV_20GeV_epic_craterlake (new artifact) (ref artifact)
• sim_dis_10x100_minQ2=1000_epic_craterlake (new artifact) (ref artifact)
• sim_dis_10x100_minQ2=1_epic_craterlake (new artifact) (ref artifact)
• sim_dis_18x275_minQ2=1000_epic_craterlake (new artifact) (ref artifact)
• sim_dis_18x275_minQ2=1_epic_craterlake (new artifact) (ref artifact)
• sim_dis_5x41_minQ2=1_epic_craterlake (new artifact) (ref artifact)
• sim_e_1GeV_20GeV_epic_craterlake (new artifact) (ref artifact)
• sim_pi_1GeV_20GeV_epic_craterlake (new artifact) (ref artifact)
  ^{_{Last updated 2026-05-15T05:56-04:00 901a1d3}}

[wdconinc]

This fails due to mixing of gcc libstdc++ and llvm libc++ symbols.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

Suppressed comments (1)

containers/debian/Dockerfile:313

• The comment above says both gcc-ar and gcc-ranlib should be used for LTO-safe archives, but the yq mutation only sets GCC_AR. If ranlib is invoked during archive creation/indexing, builds can still hit the same LTO plugin issue. Set GCC_RANLIB alongside GCC_AR in the external GCC environment block.

      "set": {
        "GCC_AR": "gcc-ar"
      }

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Suppressed comments (1)

spack-packages.sh:53

• The new ROOT cherry-pick description line starts with $$ instead of the established ## comment prefix. Because this line is inside the SPACKPACKAGES_CHERRYPICKS_FILES heredoc, it will be treated as data (not a comment) and can break downstream parsing that expects only ## ... comment lines plus the optional hash: files mappings.

## 438abd1e09f0cea9cb0ccc7e0326adcd0408196b: herwig3: add CT14(?n)lo pdfsets as build-time resources
## 4dc856a13e9066ca3249a593d351af8cda521fa3: root: add v6.40.00, v6.40.02
$$ e41e172d579e30881556873fe32f8408f6647800: root: allow ~builtin_llvm with 6.40 and newer

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Suppressed comments (2)

containers/debian/Dockerfile:325

• The yq edit for LLVM externals assumes .packages.llvm.externals[] exists; if spack external find llvm doesn’t register a system LLVM (e.g., missing llvm-config/dev package or discovery changes), this fails with a low-signal jq error. Since root~builtin_llvm relies on a usable external LLVM, fail with an explicit diagnostic so build failures are easier to interpret.

yq -iy '.packages.llvm.externals[] |= (
  .spec += " targets=all" |
  .extra_attributes += {
    "environment": {
      "set": {

containers/debian/Dockerfile:316

• The yq edit for GCC externals assumes .packages.gcc.externals[] exists; if spack external find --not-buildable gcc doesn’t populate an externals list (e.g., discovery failure or format change), this will fail with a cryptic jq error (Cannot iterate over null). Wrap the yq call to emit a clear diagnostic before failing the build, so troubleshooting is actionable.

This issue also appears on line 321 of the same file.

yq -iy '.packages.gcc.externals[] |= (
  .extra_attributes += {
    "environment": {
      "set": {
        "GCC_AR": "gcc-ar"