When mongodb/libbson#208 defined a new BSON_HAVE_RAND_R constant for CDRIVER-2407, it only added the necessary check to define this constant in CMake. The check is never performed for Autotools.

I didn't catch this on Linux builds, as the compiler seems to allow @BSON_HAVE_RAND_R@, but Windows raised an error when compiling with VC11.