Per this comment at CDRIVER-2083, there is an issue with libbson's installed CMake files, which manifests in a linker failure on the RHEL 5.5 builder when attempting to statically link against libbson.

This issue would have been caught by the C driver test suite, but the C driver team has removed the RHEL 5.5 builder from their testing matrix and has no plans to add it back. The C++ driver should only be testing against platforms that the C driver is testing on; to prevent the C++ driver team from needing to spend time investigating C driver issues on the RHEL 5.5 builder in the future, we should remove it.