use target_compile_features to use c++11 if cmake > 3.8
use target_compile_features to use c++11 if cmake > 3.8

If target_compile_features is available and cxx_std_11.
This fix compilation with clang and gcc when c++11 isn't specified by user.

antidotcb/googletest,antidotcb/googletest,empiredan/googletest,empiredan/googletest,google/googletest,empiredan/googletest,google/googletest,google/googletest,gianricardo/googletest,antidotcb/googletest,antidotcb/googletest,gianricardo/googletest,google/googletest,gianricardo/googletest