As suggested by Arthur: the threads and threads::shared

As suggested by Arthur: the threads and threads::shared
modules are always present.  This tries to preempt two
kinds of bug reports: (1) "the Perl 5.8.0 was supposed
to have this new fancy threads implementation but my Perl
doesn't seem to have it" (2) someone attempting to install
threads using CPAN.pm and CPAN.pm helpfully starting to
install perl 5.8.0 (which they might already have installed).

p4raw-id: //depot/perl@15972