The GNU 4.8 standard library's cstdio header is not aware that C++14
honors C11's removal of "gets" from stdio.h and results in an error:

  /.../include/c++/4.8/cstdio:120:11: error: no member named 'gets' in the global namespace

Detect this problematic case and default to using C++11 instead of
C++14 for building CMake itself.