I can't build libbson from mongo-c-driver 1.20.0 on Mac OS X 10.6.8 with Apple clang version 1.7 (tags/Apple/clang-77) (based on LLVM 2.9svn) from Xcode 3.2.6. The error is:

src/libbson/src/bson/bson-context.c:234:4: warning: implicit declaration of function 'arc4random_buf' is invalid in C99 [-Wimplicit-function-declaration]
   arc4random_buf (&result, sizeof (result));
   ^
1 warning generated.

Undefined symbols for architecture i386:
  "_arc4random_buf", referenced from:
      __bson_context_init_random in bson-context.c.o
ld: symbol(s) not found for architecture i386

This problem was introduced in https://github.com/mongodb/mongo-c-driver/pull/723 by unconditionally using arc4random_buf on macOS. arc4random_buf was introduced in Mac OS X 10.7.

This made me look at the code in _get_rand which raised a few questions:

1. Why is a potentially inaccurate list of arc4random_buf-supporting systems being maintained in this file? Wouldn't it be simpler and better to detect the availability of arc4random_buf at configure time?
2. If rand_r is not defined but arc4random_buf is, why does it use rand instead of arc4random_buf? Shouldn't arc4random_buf be the first choice if it is available?

Assuming the answer to both questions is that there is no good reason, I propose the following modification to address these issues:

https://github.com/macports/macports-ports/blob/36d680300e67a74055138a067e4174c88ed96793/devel/mongo-c-driver/files/arc4random_buf.patch