I think it's a bug, but so many warning discourage to post on jira, so...

while using latest ant from alpha of tools (3.0alpha4a)
i found that the class attribute on a many-to-one associationis required even if said optionnal in the doc.

if you do not, you end with the following error :
BUILD FAILED
build_hbm.xml:7: Failed in building configuration when adding src\secu.hbm.xml

hence :
...
<many-to-one name="idGroupSuper" column="idGroupSuper" class="Group" not-null="false"/>
...
is ok
but
...
<many-to-one name="idGroupSuper" column="idGroupSuper" not-null="false"/>
...
is not,

hope this will save some hour to others