Revert C11 compilation requirement

[dagostinelli]

No description provided.

[chapterjason]

+1

[richardeigenmann]

Using nullptr on line 216 of b2Body.cpp appears to be C++11 which will not compile on older standards.

Whilst it is important for a library to remain usable in older applications time does move on and eventually upgrades should be made by all to the latest current standard. Teams with a valid reason to stay behind can continue to use the old versions of the library.

I would vote to add the following line to the main CMakeLists.txt and keep the nullptr:
set (CMAKE_CXX_STANDARD 11)

But if compatibility with C++ from 10 years ago is a huge issue let's just approve this pull request and leave it up to the users of the library if they want to use C++11 or not.

I found the views of Bjarne interesting that are quoted in this article: http://stackoverflow.com/questions/20509734/null-vs-nullptr-why-was-it-replaced

[louis-langholtz]

Please note that nullptr made it into other files also. See Box2D/Testbed/Framework/Main.cpp for another instance. This pull request only addresses the C++11 nullptr in Box2D/Box2D/Dynamics/b2Body.cpp however.

[erincatto]

I plan to use C++11 for other things, like override. If you cannot support C++11, I recommend making a fork.