Support --c++11 for libmongoclient

This commit adds support for building libmongoclient in C++11 mode.  It requires applying a trivial patch from the current HEAD of the 26compat branch of the repository which will be included in the upcoming 2.6.1 release (see https://jira.mongodb.org/browse/CXX-196 and https://github.com/mongodb/mongo-cxx-driver/pull/61 for details).

This commit 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 (Why min version 10.8?  Why explicitly pass the libc++ flag when it is the default?), 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