See svn commit 188894 by unhammer, which adds target definitions below the include line (forbidden teritory), because that is the only possible location: the variable being extended is defined in the included file, and is thus not yet available within the area set asite for local additions.
Must be fixed before we move to github, and should be easy to fix.
This also goes for the variables
in tools/mt/Makefile.am / am-shared/tools-mt-dir-include.am
Fixed in rev 189372, 189374-189376, 189378, 189380 and 189382.
This is at the same time a beginning of fixing bug #3658.