Bump CMake files to at least c++11.

Removed all configurations that explicitly test or set the c++ standard flags. The only place the standard is now configured is at the top of the main CMakeLists.txt file, which can easily be updated (e.g. if we decide to move to c++14+). This can also be set via command-line using

> cmake -DCMAKE_CXX_STANDARD 14

Kept the EIGEN_TEST_CXX11 flag for now - that still controls whether to build/run the cxx11_* tests. We will likely end up renaming these tests and removing the CXX11 subfolder.