Fix suppression of C++11 propagation warning.

- Since the update to optionally use target_compile_features(), the
  warning about Ceres propagating C++11 compile flag requirements to
  clients was suppressed dependent upon the compiler option selected by
  CMake to satisfy the C++11 requirements for Ceres (e.g. if using
  -std=gnu++11 instead of -std=c++11).
- Now we display the warning for all CMake versions where any C++11
  related flags can be exported in the Ceres target (CMake >= 2.8.12)
  if Ceres was compiled with the CXX11 option enabled.

Change-Id: I5cb91e773fc7c41996b5eabadcaa295ebd7de4f7