lxession-0.4.9.2 currently uses VALA_MAX_API_VERSION="0.20". LFS has no such restriction (see http://www.linuxfromscratch.org/blfs/view/svn/lxde/lxsession.html). Please bump vala to 0.22.
Thanks for your report. Updated VALA_MAX_API_VERSION to 22.
(In reply to Pavel Kazakov from comment #1) > Thanks for your report. > > Updated VALA_MAX_API_VERSION to 22. s/22/0.22/