At the mongo C driver git page, one suggestion is to download source from github and compile it.

But it doesn't suggest which branch to use, and of the logical choices (master, r1.2, ubuntu), the configure script finishes with a warning not to use the code, that the API/ABI might change, that this is just for test purposes.

build/autotools/PrintBuildConfiguration.m4

Presumably there exists some branch or tag that signifies the latest stable release. Perhaps the documentation might say so.