module for ensuring that a platform has weaken support

 One recurring problem in modules that use Scalar::Util's weaken function is
 that it is not present in the pure-perl variant.
 .
 While this isn't necessarily always a problem in a straight CPAN-based Perl
 environment, some operating system distributions only include the pure-Perl
 versions, don't include the XS version, and so weaken is then "missing" from
 the platform, despite passing a dependency on Scalar::Util successfully.
 .
 The normal solution is to manually write tests in each distribution to ensure
 that weaken is available.
 .
 Task::Weaken restores the functionality testing to a dependency you do once
 in your Makefile.PL, rather than something you have to write extra tests for
 each time you write a module.