[openbsd] Add OpenBSD CI job to GitHub Actions

.builds/openbsd.yml last produced a passing result on 2023-11-10.  The
job after that, #1520293 on 2025-06-28, failed, and nothing has run on
builds.sr.ht since; the SourceHut badge in README currently reads
"builds: failure".  No sr.ht context appears on any of the last 19
commits here either, so even when those builds did run, their result was
never visible on a commit or a pull request in this repository.

This adds a job that runs the build in an OpenBSD 7.9 VM on a
GitHub-hosted runner:

  autotools  autoreconf -vif; configure; make
  legacy     ./Configure -n openbsd; make; bash ./check.bash openbsd

libtool has to come from packages.  OpenBSD's base /usr/bin/libtool is a
different program and brings no GNU libtool m4 files, so without the
package aclocal cannot find LT_INIT and autoreconf stops at

    Makefile.am:100:   you'll need to arrange for the Libtool m4 files
    Makefile.am:100:   to be found by aclocal
    autoreconf-2.72: error: automake failed with exit status: 1

after which the half-written configure dies with

    configure[3882]: syntax error: `disable-fast-install' unexpected

.builds/openbsd.yml installs libtool for the same reason.  autoconf and
automake are pinned to a package version because pkg_add reports both
names as ambiguous -- 7.9 packages eighteen autoconf versions and twelve
automake versions -- and AUTOCONF_VERSION / AUTOMAKE_VERSION have to
agree with whichever is installed.

"make check" is deliberately not run on the autotools build.  It fails
tests/LTbasic2 there with "opened regular file wasn't found"; that test
is not part of the legacy suite, and the two paths end up with very
different flag sets, since configure.ac's openbsd arm sets only
LSOF_VERS while Configure also sets HASIPv6, HAS_SYS_PIPEH and
HAS_STRFTIME.  That is left for a separate change.

Verified on GitHub Actions, OpenBSD 7.9:

    bash ./check.bash openbsd
    successful: 14
    skipped: 0
    failed: 0
    All 14 test cases are passed successfully

Author: neilpang

.github/workflows/build.yml

@@ -102,3 +102,39 @@ jobs:
           ./Configure -n freebsd
           make
           bash ./check.bash freebsd
+
+  openbsd:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v7
+    - name: build and test in an OpenBSD vm
+      uses: vmactions/openbsd-vm@v1
+      with:
+        release: "7.9"
+        usesh: true
+        copyback: false
+        prepare: |
+          pkg_add -I bash git libtool groff autoconf-2.72p0 automake-1.18.1
+        run: |
+          set -e
+          src=$(pwd)
+          export AUTOCONF_VERSION=2.72
+          export AUTOMAKE_VERSION=1.18
+
+          # autotools path: build only.  "make check" additionally fails
+          # tests/LTbasic2 here with "opened regular file wasn't found";
+          # that test is not part of the legacy suite below and is left
+          # for a separate fix.  libtool has to come from packages -- the
+          # base /usr/bin/libtool is a different program and aclocal
+          # cannot find LT_INIT without the package's m4 files.
+          autoreconf -vif
+          mkdir /tmp/build
+          cd /tmp/build
+          "$src"/configure
+          make
+          cd "$src"
+
+          # legacy Configure, the path .builds/openbsd.yml covered
+          ./Configure -n openbsd
+          make
+          bash ./check.bash openbsd