Fix test/TDataXtd_test/CMakeLists.txt

It seems commit bc50e52 had been reverted by commit e9ad629, no idea
what happened.  Anyway, reapply bc50e52:

    Run TDataXtd_test only if OCE_DISABLE_X11 option is not set

    It depends on TKCAF and TKXCAF modules which are built only
    if OCE_DISABLE_X11 is not set.

    Moreover TKXCAF is built by DATA_EXCHANGE and not OCAF, so
    we must check against DATA_EXCHANGE instead of OCAF.
Merge pull request #342 from tpaviot/review/fix-TDataXtd_test-dependency

Fix test/TDataXtd_test/CMakeLists.txt