BUG: arpack-ng: fix bug occurring on OSX or LAPACK 3.2.1

See discussion at http://forge.scilab.org/index.php/p/arpack-ng/issues/1259/

The Ritz vector purification step assumes workl(iq) still contains the
original Q matrix. This is however overwritten by the call to xGEQR2
earlier.

This patch fixes the issue by making a copy of the last row of the eigenvector
matrix, after it is recomputed after QR by xORM2R. The work space
WORKL(IW+NCV:IW+2*NCV) used for the TAU values of the QR decomposition is not
used later in the routine, and can be used to store a copy of the last row.

Fixes gh-2547 (provided you use -ff2c flag for compiling)

Thanks to Michael Wimmer for tracing the issue.
Merge pull request #2684 from pv/arpack-fix

BUG: arpack-ng: fix bug occurring on OSX or LAPACK 3.2.1