Add option to use C++11 (not TR1) shared_ptr & unordered_map.

- On at least some compilers, -std=c++11 is required in order to compile
  against std::shared_ptr & std::unordered_map, which resulted in our
  checks failing to find them and using the TR1 versions instead, which
  causes conflicts for users using C++11.
- Now, if the compiler supports it and the user enables the CXX11
  option, we explicitly enable C++11 before searching for shared_ptr &
  unordered_map, which means we should always find the C++11 versions
  if they are available.
- As use of CXX11 results in a version of Ceres that must be used with
  -std=c++11 for GCC & Clang, we roll this into the Ceres target when
  the version of CMake supports this, otherwise we warn the user they
  will have to do this themselves.
- CXX11 is OFF by default, to ensure that the behaviour of Ceres is
  unchanged from before.

Change-Id: I157ea7a4fadc6bc02da176b8e771f1f327ccaf78