https://abi-laboratory.pro/tracker/objects_report/mongo-c-driver/1.4.2/current/report.html
Did we inadvertently change the soname, perhaps in CDRIVER-1347? Also check on libbson.
Update: CMake builds create sonames like "libmongoc-1.0.so.1" but the Autotools create sonames like "libmongoc-1.0.so.0". Presumably these should be the same.