613e01afccbd4efcf002af5837897b30bb3b548a configure script: fix bug in Octave detection

================================ DYNARE MAKE CHECK MATLAB RESULTS ================================ | TOTAL: 199 | PASS: 198 | FAIL: 0 | XFAIL: 1 | XPASS: 0

================================ DYNARE MAKE CHECK OCTAVE RESULTS ================================ | TOTAL: 197 | PASS: 196 | FAIL: 0 | XFAIL: 1 | XPASS: 0

A full log can be found on karaba in '/tmp/dynare-master-check-QdzPPEJBw0.log'.