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
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.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>