GHA/linux-old: add an autoconf/automake build

This verifies that using older autotools still works. The results of the
autoconf build should end up being identical to the second of the two
cmake builds (the difference in the platform string notwithstanding).

Closes #14816

.github/workflows/linux-old.yml

@@ -53,7 +53,7 @@ env:
 
 jobs:
   cmake:
-    name: linux (cmake)
+    name: linux (cmake & autoconf)
     runs-on: 'ubuntu-latest'
     container: 'debian:stretch'
 
@@ -69,7 +69,7 @@ jobs:
           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 libssh2-1-dev libc-ares-dev heimdal-dev libldap2-dev stunnel4 groff
+          apt-get install -y --no-install-suggests --no-install-recommends cmake make automake autoconf libtool gcc pkg-config libpsl-dev libzstd-dev zlib1g-dev libssl1.0-dev libssh-dev libssh2-1-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+deb10u4_amd64.deb
@@ -94,16 +94,36 @@ jobs:
             -DENABLE_ARES=ON -DCURL_ZSTD=ON -DCURL_USE_GSSAPI=ON -DCURL_USE_LIBSSH2=OFF -DCURL_USE_LIBSSH=ON \
             -DCURL_LIBCURL_VERSIONED_SYMBOLS=ON
 
-      - name: 'build'
+      - name: 'cmake build'
         run: |
           make -C bld-cares
           bld-cares/src/curl --disable --version
 
-      - name: 'install'
+      - name: 'cmake install'
         run: make -C bld-cares install
 
-      - name: 'build tests'
+      - name: 'cmake build tests'
         run: make -C bld-cares testdeps
 
-      - name: 'run tests'
+      - name: 'cmake run tests'
         run: make -C bld-cares test-ci
+
+      - name: 'autoreconf'
+        run: autoreconf -if
+
+      - name: 'configure (out-of-tree, libssh2)'
+        run: |
+          mkdir bld-am
+          cd bld-am
+          ../configure --disable-dependency-tracking --enable-warnings --enable-werror --with-openssl --enable-ares --with-libssh --with-zstd --with-gssapi --prefix="$PWD"/../install-am
+
+      - name: 'autoconf build'
+        run: |
+          make -C bld-am
+          bld-am/src/curl --disable --version
+
+      - name: 'autoconf install'
+        run: make -C bld-am install
+
+      - name: 'autoconf build tests'
+        run: make -C bld-am/tests all