CI: Add OpenBSD build tests for staged commits (#1964)

Use a GitHub-hosted VM to create OpenBSD test environment.

We have not enabled ccache optimization for OpenBSD tests because we
do not know how to copy updated ccache files from VM back into runner.

.github/workflows/slow.yaml

@@ -181,3 +181,61 @@ jobs:
         with:
           name: build-logs-freebsd-${{ matrix.osversion }}
           path: btlayer-*.log
+
+  openbsd:
+    runs-on: ubuntu-22.04
+
+    steps:
+      - name: Checkout Sources
+        uses: actions/checkout@v4
+
+      - name: Run test-builds
+        id: test-builds
+        uses: vmactions/openbsd-vm@v1
+        with:
+          usesh: true
+          prepare: |
+            pkg_add \
+              autoconf-2.72p0 \
+              autoconf-archive \
+              automake-1.16.5 \
+              bash \
+              coreutils \
+              cppunit \
+              git \
+              ggrep \
+              gmake \
+              libltdl \
+              libtool \
+              libnettle \
+              libxml \
+              libtool \
+              m4 \
+              metaauto
+
+            mkdir $HOME/bin
+            ln -s /usr/local/bin/ggrep $HOME/bin/grep
+
+          run: |
+            export MAKE=gmake
+            export pjobs="-j`gnproc`"
+            export AUTOMAKE_VERSION=1.16
+            export amver=${AUTOMAKE_VERSION}
+            export ACLOCAL_AUTOMAKE_DIR="/usr/local/share/aclocal-${AUTOMAKE_VERSION}"
+            export ACLOCAL_PATH="/usr/local/share/aclocal:/usr/local/share/aclocal-${AUTOMAKE_VERSION}"
+            export AUTOCONF_VERSION=2.72
+            export acver=${AUTOCONF_VERSION}
+            export ltver=2.4.2
+            export CFLAGS='-Wno-compound-token-split-by-macro'
+            export LDFLAGS="-L/usr/local/lib"
+            # until we remove GNUisms from our grep commands,
+            # shadow system `grep` with `ggrep` installed/linked earlier
+            export PATH="$HOME/bin:$PATH"
+            ./test-builds.sh
+
+      - name: Publish build logs
+        if: success() || failure()
+        uses: actions/upload-artifact@v4
+        with:
+          name: build-logs-openbsd
+          path: btlayer-*.log