cmake: auto-detect `strtonum` Notice that just like in autotools, this detection also doesn't take into account the targeted OS version. Meaning it detects `strtonum` even if targeting e.g. macOS older than release v11 Big Sur (which introduced this funcitions), if the SDK declares it. Wrong detection will either cause a binary broken on older macOS and/or trigger compiler warnings. Ref: https://github.com/libressl/portable/issues/928#issuecomment-1850178282 Ref: https://github.com/libressl/portable/issues/928#issuecomment-1850276298 Prerequisite: https://github.com/libressl/portable/issues/928#issuecomment-1850356408