[autotools] Fix wildcard stuff in doc/handbook.

Make complained about an empty variable. Probably it did
not like the :=+, thus it was replaced by " \" and
everything is added in one line.
(reviewed by fetzer)


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@13341 2fb0f335-1f38-0410-981e-8018bf24f1b0