The libbson and libmongoc build processes create several files that are not recorded in .gitignore. Please add them, so that projects that use them as submodules don't see these files as "dirty" in the specific submodules.
For example:
build/cmake/libbson-1.0-config-version.cmake build/cmake/libbson-1.0-config.cmake build/cmake/libbson-static-1.0-config-version.cmake build/cmake/libbson-static-1.0-config.cmake
In previous versions, libmongoc had similar issues, so please check that too.