Support --c++11 for libmongoclient

This commit adds support for building libmongoclient in C++11 mode.

It 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>

Changes since v1:
- Pass --osx-version-min unconditionally and use MacOS.version instead
  of 10.7 (min version required for libc++) at @jacknagel's request.
- Pass --libc++ for Mavericks even for C++11 mode when it results in
  redundant compiler flags for better readability at @jacknagel's
  request.
libmongoclient: add 2.6.1 bottle.