We currently use --with-libbson and --with-libmongoc to switch from using bundled versions of the libraries to system ones. However, these switches need to use the same value; compiling with system libbson and bundled libmongoc is not supported. To avoid adding a third switch for libmongocrypt, we re-use the --with-libbson value when deciding how to compile libmongocrypt.

To future-proof these switches, the new --with-system-libs (default no) will take precedence over the old --with-libbson and --with-libmongoc switches. These can be removed in a 2.0 release.