CI: Add CI build on Debian stretch to test old support

This version still has ELTS support and contains some old versions of
key components like cmake to help prevent us from breaking that support.

Closes #13029

.github/workflows/linux-old.yml

@@ -0,0 +1,95 @@
+# Copyright (C) Daniel Fandrich, <dan@coneharvesters.com>, et al.
+#
+# SPDX-License-Identifier: curl
+#
+# Compile on an old version of Linux that has barely the minimal build
+# requirements for CMake. This tests that curl is still usable on really
+# outdated systems.
+#
+# Debian stretch is chosen as it closely matches some of the oldest major
+# versions we support (especially cmake); see docs/INTERNALS.md and it
+# is still supported (as of this writing).
+# stretch has ELTS support from Feexian until 2027-06-30
+# For ELTS info see https://www.freexian.com/lts/extended/docs/how-to-use-extended-lts/
+# The Debian key will expire 2025-05-20, after which package signature
+# verification may need to be disabled.
+# httrack is one of the smallest downloaders, needed to bootstrap ELTS,
+# and won't conflict with the curl we're building.
+
+name: Old Linux
+
+on:
+  push:
+    branches:
+      - master
+      - '*/ci'
+    paths-ignore:
+      - '**/*.md'
+      - '.azure-pipelines.yml'
+      - '.circleci/**'
+      - '.cirrus.yml'
+      - 'appveyor.*'
+      - 'packages/**'
+      - 'plan9/**'
+      - 'projects/**'
+      - 'winbuild/**'
+  pull_request:
+    branches:
+      - master
+    paths-ignore:
+      - '**/*.md'
+      - '.azure-pipelines.yml'
+      - '.circleci/**'
+      - '.cirrus.yml'
+      - 'appveyor.*'
+      - 'packages/**'
+      - 'plan9/**'
+      - 'projects/**'
+      - 'winbuild/**'
+
+permissions: {}
+
+env:
+  MAKEFLAGS: -j 3
+  DEBIAN_FRONTEND: noninteractive
+
+jobs:
+  cmake:
+    name: linux (cmake)
+    runs-on: 'ubuntu-latest'
+    container: 'debian:stretch'
+
+    steps:
+      - name: 'install prereqs'
+        # Remember, this shell is dash, not bash
+        run: |
+          sed -E -i -e s@[a-z]+\.debian\.org/@archive.debian.org/debian-archive/@ -e '/ stretch-updates /d' /etc/apt/sources.list
+          apt-get update
+          # See comment above if this fails after 2025-05-20
+          apt-get install -y --no-install-suggests --no-install-recommends httrack
+          httrack --get https://deb.freexian.com/extended-lts/pool/main/f/freexian-archive-keyring/freexian-archive-keyring_2022.06.08_all.deb
+          dpkg -i freexian-archive-keyring_2022.06.08_all.deb
+          echo 'deb http://deb.freexian.com/extended-lts stretch-lts main contrib non-free' | tee /etc/apt/sources.list.d/extended-lts.list
+          apt-get update
+          apt-get install -y --no-install-suggests --no-install-recommends cmake make gcc pkg-config libpsl-dev libzstd-dev zlib1g-dev libssl1.0-dev libssh-dev libc-ares-dev heimdal-dev libldap2-dev stunnel4 groff
+          # GitHub's actions/checkout needs a newer glibc. This one is the
+          # latest available for buster, the next stable release after stretch.
+          httrack --get https://security.debian.org/debian-security/pool/updates/main/g/glibc/libc6_2.28-10+deb10u2_amd64.deb
+          dpkg -i libc6_2.28-10+deb10u2_amd64.deb
+
+      - uses: actions/checkout@v4
+
+      - name: 'cmake generate out-of-tree'
+        run: |
+          mkdir build
+          cd build
+          cmake -DBUILD_SHARED_LIBS=ON -DENABLE_ARES=ON -DCURL_ZSTD=ON -DCURL_USE_LIBSSH=ON -DCURL_USE_GSSAPI=ON ..
+
+      - name: 'build'
+        run: make -C build
+
+      - name: 'install'
+        run: make -C build install
+
+      - name: 'tests'
+        run: make -C build test-ci