missing `_concat_fortran_string`

deal with the dependence to calculate eigenvalues and vectors. get rid of `quadmath` dependency and build for all platforms.

Start by locating the eigenvalue and eigenvector calculation path and the missing `_concat_fortran_string` symbol, then inspect where the `quadmath` dependency enters the build. Done means the dependency is removed and the project builds for all supported platforms.

Written by the indexing model from the issue text.