Use target_compile_features() to specify C++11 requirement if available.

- Use target_compile_features() to specify the C++11 dependency for
  Ceres if the CXX11 option is enabled and the current CMake version
  supports it (>= 3.1).  Otherwise fall back onto our existing
  target_compile_options() solution if available.
- We prefer the use of target_compile_features() if available as it more
  gracefully handles ‘upgrading’ of the C++ standard in client projects
  that depend upon Ceres, e.g. if the client requires C++14.  The
  current solution may fail to produce the expected result in this case
  as raised in
  https://github.com/ceres-solver/ceres-solver/issues/273.

Change-Id: Ib3cff8d4b9fe93fa6d6b376b4dd53923bb1c4ecc