Fix an OSD_Path constructor on unknown systems

See https://buildd.debian.org/status/fetch.php?pkg=oce&arch=hurd-i386&ver=0.11-2&stamp=1355676025
several tests fail on Hurd:
    24 - IGESImportTestSuite.testImportIGES_1 (Failed)
    25 - STEPImportTestSuite.testImportAP203_1 (Failed)
    26 - STEPImportTestSuite.testImportAP203_2 (Failed)
    27 - STEPImportTestSuite.testImportAP214_1 (Failed)
    28 - STEPImportTestSuite.testImportAP214_2 (Failed)
    29 - STEPImportTestSuite.testImportAP214_3 (Failed)
    34 - TDataXtdTestSuite.testPattern (Failed)
    35 - UnitsAPITestSuite.testCurrentUnits (Failed)
    36 - UnitsAPITestSuite.testMDTVCurrentUnits (Failed)
because this OSD_Path constructor did nothing when system is unknown.
Use UnixExtract by default.