libmongoc's "make install" copies two dozen private header files into the system include path, which encourages developers to depend on private, unstable API/ABI:

https://github.com/mongodb/mongo-c-driver/blob/1.2.1/src/mongoc/Makefile.am#L19

libmongoc should omit private headers from the installed files, like libbson does:

https://github.com/mongodb/libbson/blob/1.2.1/src/bson/Makefile.am#L8