Better tests, plus Perl 5.10.0 support. :)

The attached patchset:

* Improves test output by using `is()` rather than `ok()` where appropriate.
* Causes the `02noenv.t` tests to pass on my systems, by testing to see if environment variable are defined rather than existing.  (`local` in the test cases leaves them in existence.)
* Tweaks build and feature lines to allow installation on 5.10.0+ systems (upon which it works great, as I've just tested it.)

Many thanks!

  Paul