mongodb_log: include mongoclient in the LDFLAGS
mongodb_log: provide the MongoDB version as macro

The MongoDB header files don't include any version information and
therefore make conditional compilation impossible. As a workaround, we
query the mongo binary for its version number and provide the output as
pre-processor macros. Unfortunately, this approach does not seem to be
very robust.
refbox: adapt for newer MongoDB versions

Starting from version 2.2.x the BSONObjBuilder uses "verify" instead of
"assert" to validate that correct arguments are passed to its member
functions. This results in different exceptions being thrown in case of
a problem:
* Version 2.0.x: bson::assertions
* Version >=2.2.x: mongo::AssertionException

With his commit the correct exception is selected based on the MongoDB
version.