Trying to compile mongo-cxx-driver on FreeBSD fails with error
#error Must implement SecureRandom for platform

The OpenBSD implementation for SecureRandom should work on FreeBSD as well (not checked, as I have no direct access to a FreeBSD installation at the moment).

You can see a complete log of the failure here:
https://buildd.debian.org/status/fetch.php?pkg=mongo-cxx-driver-legacy&arch=kfreebsd-amd64&ver=1.1.1-1&stamp=1464681161