OpenSceneGraph 3.1.2 clang c++11 compiler support

Added an option to compile with clang using libc++ rather than libstdc++:

--with-c++11

For example,

brew install open-scene-graph --with-c++11

Without using this option when installing open-scene-graph, a application compiled with libc++ rather than libstdc++ will crash when trying to set the string name of an osg::Node.

Also fixed a bug in the Uniform header: the set method from the base class used in the constructor of the various matrix templates is hidden by set methods in the derived classes. Added "using base_class::set", like in OSG 3.1.3.