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.

Closes #29003.

[jn: style fixes]

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Signed-off-by: Jack Nagel <jacknagel@gmail.com>