GH #87, PR #88 - Fix configure-time function detection probes being broken by compiler optimization. The probes stored function pointers in local variables which -O2/-Os (added to probe flags in PR #81) eliminated as dead stores, so the linker never saw the function reference. On systems where openpty() lives in -lutil (older glibc, BSDs), the probe falsely succeeded without -lutil, producing "undefined symbol: openpty" at runtime. Fixed by storing the function pointer in a file-scope global variable which the optimizer cannot eliminate.
Maintenance:
PR #89 - Add Ubuntu LTS version matrix (20.04, 22.04, 24.04) to the GitHub Actions test suite. Exercises the system perl on each current Ubuntu LTS release via Docker containers, running after the main ubuntu job.