Fix test failures on unthreaded perls on FreeBSD
When running tests, display the gtk+ version
Use Glib::Object::Introspection->CHECK_VERSION in the tests

Instead of executing pkg-config.
Tighten the GtkRadio* test skip conditions

Require the gtk+ version that I actually tested things against, not just the
first release in the corresponding minor series.  This hopefully fixes some
failure reports from CPAN testers.
Release v0.020