ci: Wire up OS/2 builds to GitHub Actions.

.github/workflows/os2.yml

@@ -0,0 +1,22 @@
+name: Build (OS/2)
+
+on: [push, pull_request]
+
+jobs:
+  os2:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: open-watcom/setup-watcom@v0
+      - name: Build SDL2
+        run: |
+          wmake -f Makefile.os2
+      - name: Build tests
+        run: |
+          cd test && wmake -f Makefile.os2
+          cd ..
+      - name: distclean
+        run: |
+          wmake -f Makefile.os2 distclean
+          cd test && wmake -f Makefile.os2 distclean
+          cd ..