See https://github.com/abseil/abseil-cpp/issues/259 for context.
This change introduces the ABSL_PROPAGATE_CXX_STD option (default to OFF for now, though it prints a warning in CMake 2.8+ builds that a future Abseil release will default to ON). When enabled, all Abseil CMake targets will set the cxx_std_11 target meta feature (which will then propagate to targets that depend upon Abseil) rather than setting the target-level CXX_STANDARD property to CMAKE_CXX_STANDARD (which is the default value anyway), which doesn't propagate.
Updates README documentation to clarify behavior and with a different example recommendation for library projects (which should generally leave CMAKE_CXX_STANDARD to the root application project).
See https://crascit.com/2015/03/28/enabling-cxx11-in-cmake/ for a useful overview of these different CMake features surrounding C++ standard version configuration.
