Do not build shared libs if not supported

What does this implement/fix?

CMake complains with an error when Eigen is added on platforms that do not support shared libraries (such as ARM Cortex). This queries whether the target platform can build shared libraries via get_cmake_property and adds a new flag, EIGEN_BUILD_SHARED_LIBS that is available to subdirectories. The eigen_blas and eigen_lapack targets are only added if this flag is true. The targets eigen_blas_static and eigen_lapack_static are added regardless.