front-end: compilation fails - how to fix 'use poisoned USE_MAPPED_LOCATION'

[Lemaitre Laurent-R29173]

Hi All,
I am building a new gcc front-end based on the treelang example.
I recently updated my gcc local SVN repository.
When I compile my front-end I get the following errors:
../../svn/gcc/laurent/lex.l:6:8: error: attempt to use poisoned
"USE_MAPPED_LOCATION"
../../svn/gcc/laurent/lex.l:26:8: error: attempt to use poisoned
"USE_MAPPED_LOCATION"
../../svn/gcc/laurent/lex.l: In function 'update_locus':
../../svn/gcc/laurent/lex.l:42: error: lvalue required as left operand
of assignment
../../svn/gcc/laurent/lex.l:60:11: error: attempt to use poisoned
"USE_MAPPED_LOCATION"
I used to refer to the treelang example in order to adapt my front-end
and make
it compile with the latest SVN version of gcc.
However I just found out that treelang is no more maintained (since a
few months).
Is-there a good (wiki) place where I can get some info on how to fix my
issue with 
USE_MAPPED_LOCATION?
Thanks for your help!
Laurent