I'm currently trying to package libmongocrypt for Gentoo, and I'm having some issue with using the system bson instead of the localy built: cmake/ImportBSON.cmake loads both static and dynamic library regardless of the value -DUSE_SHARED_LIBBSON. This disallow building with a shared-only libbson (such as when building it with -DENABLE_STATIC=OFF).

The repro I'm using (Gentoo w/ dev-libs/libbson[-static-libs]), but as that might not be your system, you can trigger another quite related error by only keeping -DMONGOCRYPT_MONGOC_DIR=USE-SYSTEM

#!/bin/sh -eu
cmake \
    -DENABLE_ONLINE_TESTS=OFF \
    -DMONGOCRYPT_MONGOC_DIR=USE-SYSTEM \
    -DUSE_SHARED_LIBBSON=ON
make stubbed-crypt_shared # faster than full make

IMO the issue arises from a misuse of MONGOCRYPT_MONGOC_DIR; you can see the problem iff its value is USE-SYSTEM but the other flags aren't set: it will default to local build and add_subdirectory("USE-SYSTEM") which is clearly wrong.

I've managed to put a PR fixing that at https://github.com/mongodb/libmongocrypt/pull/681 commenting there more on the code side.