Cope with absent Perl modules threads and Thread::Queue. * automake.in: Instead of `use threads', use `require threads; import threads' inside a BEGIN section. Likewise for Thread::Queue. * lib/Automake/ChannelDefs.pm (Automake::ChannelDefs): Likewise. * lib/Automake/tests/Condition-t.pl: Likewise. * lib/Automake/tests/DisjConditions-t.pl: Likewise. * configure.ac: Likewise, use `require' and `import'. Report by Eric Blake, fix suggested by Ben Pfaff. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>