cmake should fail here if Eigen3 is not available. instead of failing during compilation with a `static_assert`
bad design decision here. you should use:
```FIND_PACKAGE(Eigen3 3.1.2 REQUIRED)``` which will fail cmake itself if eigen is not available and you want to set SetLinAlgLib to be eigen.

but again i have very big concerns having linalg backend set during configuration/compilation