Support --c++11 for libmongoclient

This is a resubmission of #28615 now that 2.6.1 has been released.  It upgrades libmongoclient to 2.6.1 and adds support for building libmongoclient in C++11 mode.

This PR also changes boost from a build dependency to a normal dependency in both C++98 and C++11 mode. Since the library can't be statically linked into a program or used without several of the boost libraries, it didn't seem reasonable to only declare boost as a build dependency. If you would prefer to consider this change separately, I can remove it and submit a different issue to discuss.

Also note that I am not completely sure why Mavericks has the special build flags that it does (and why they differ from the Mavericks-specific flags for the mongodb package), so my guess at how to reconcile these with C++11 may not be correct. Someone knowledgeable may want to double-check.

Thanks for considering,
Kevin