Commit (8d7a2b7cdd429d6d5a7853848a804b29ab619b1a) added a #define guard for the fast-path code for bson_oid_to_string that ensured it was only enabled on x86 platforms.
When I build on Windows, this fast-path code is not enabled (see the first line of bson_oid_to_string):
#if !defined(__i386__) && !defined(__x86_64__)
Neither of these macros is defined by Visual Studio.