Hi Stephane,
Something else must be happening specific to your environment. As i mentioned before, Ecilpse doesn't modify the system environment with the new TEMP and TMP values - the overridden values are only used in the scope of the Gmake child-process that CCS spawns. And as long as the directory exists, and has 'execute' privileges for your user, Gmake works as expected.
Please double check that you haven't changed the values of the TEMP and TMP variables in your system (through the Windows dialog). And make sure that the new temp directory does have 'execute' rights specifically for your user.
Thanks,
- Baltasar