Add an edg builder for the open-source EDG front end

[mattgodbolt-molty]

Companion PRs: #146 (builder), compiler-explorer/infra#2393 (installables), compiler-explorer/compiler-workflows#83 (daily build), compiler-explorer/compiler-explorer#9204 (properties).

EDG's C/C++ front end is now open source at https://github.com/edgcpp/compiler. This adds an edg builder image so we can build it ourselves, instead of EDG sending us tarballs.

edg/build.sh <tag|trunk> builds the linux-gcc-release preset plus the x86-64 runtime library, then runs EDG's own edg-pack-cpfe-ce packer for both modes. The output is one tarball:

• edg-<ver>/gcc/: GNU mode, including the experimental headers so <experimental/meta> works with --set_flag reflection
• edg-<ver>/default/: EDG's own headers only
• edg-<ver>/tools/: EDG's edg-scrape-compiler and make_predef_macro_table, which the installer runs against the backend gcc

A tag builds edg-<tag>.tar.xz; trunk builds main as edg-trunk-<date>.tar.xz and skips if the revision hasn't changed.

Two environment settings are needed:

• EDG_GCC_VER_SCRAPE, as in EDG's HACKING.md: without it their build config assumes gcc 14 and passes -Wno-error=return-mismatch, which the image's gcc 11 rejects.
• EDG_RUNTIME_LIB=edgrt: the packer expects libedgrt.a but the build names it libC.a by default. Wyatt at EDG suggested this in edg-pack-cpfe-ce expects lib/libedgrt.a but the build produces libC.a edgcpp/compiler#16.

Tested locally: docker build then build.sh 7.0 and build.sh trunk both produce tarballs, and a rerun of trunk with the current revision reports SKIPPED. Test-installing those tarballs, GNU mode compiles and runs a C++20 program, and a P2996 reflection test with --c++26 --set_flag reflection.

🤖 Generated with Claude Code

https://claude.ai/code/session_019UuPJbXuMH8A4DCDNoiBiv

[mattgodbolt]

CC @DarkArc

[mattgodbolt]

LGTM, getting robot second opinion

[Copilot]

Copilot review overview

🟢 Approval recommended

The builder, packaging layout, dependencies, and CI integration are consistent and complete.

Review effort: Balanced
Findings: None

What changed in this PR

Adds an EDG builder image for producing installable open-source EDG packages.

Changes:

• Builds tagged or trunk EDG releases with GNU/default modes and supporting tools.
• Adds the required Ubuntu build environment.
• Publishes the builder image through CI.

File	Description
edg/​build.sh	Builds and packages EDG releases.
Dockerfile.edg	Defines the EDG build environment.
.github/​workflows/​build.yml	Adds the EDG image to CI.

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[jhsedg]

That is really great! Thank you so much for that!