aa135218a5b6901d31ad560f6f6b88911459fca7 preprocessor: don't substitute 'dates' when preceded by an underscore

================================ DYNARE MAKE CHECK MATLAB RESULTS ================================ | TOTAL: 443 | PASS: 441 | FAIL: 0 | XFAIL: 2 | XPASS: 0

================================ DYNARE MAKE CHECK OCTAVE RESULTS ================================ | TOTAL: 441 | PASS: 439 | FAIL: 0 | XFAIL: 2 | XPASS: 0

A full log can be found on karaba in '/tmp/dynare-master-check-pGm9VFKTHf.log'.