Eigen requires C++14 to build

[codeinred]

When attempting to build the library with Apple Clang, the build fails with the following error:

/Users/alecto/3rd/eigen/Eigen/src/Core/util/Macros.h:709:2: error: Eigen requires at least c++14 support.
#error Eigen requires at least c++14 support.
 ^

Current versions of gcc and llvm clang use C++17 by default, so CMake doesn't bother to pass the -std=c++11 flag, which is likely why this wasn't caught previously.