strexc(3) on Linux

The text 910d0bf8c929, as Debian 2.2 (Potato), Debian 2.1 (Slink), Debian 2.0 (Hamm) shipped it.

reorder the real Schur factorization of a real matrix A = Q*T*Q**T, so that the diagonal block of T with row index IFST is moved to row ILST