Release v2.6.0

This version removes validation of language tags; dictionary names no longer
have to be valid language tags. This is because dictionaries in the wild
sometimes are sometimes not named with valid tags, and so are not visible to
Enchant. Programs that use Enchant will have to cope with this, for the
purposes of working out which languages are supported; but it should not a
big deal, as Enchant already accepted valid language tags with extra stuff
on the end, such as “en_GB-ize”.

Debug output now uses GLib’s debug system, and can be enabled at run-time by
setting the environment variable G_MESSAGES_DEBUG to ‘libenchant’. If you
have had problems understanding why Enchant is not using the spelling
provider you expect, now’s the time to get in touch via the issue tracker
and see if we can work out what’s going on!

The Aspell back-end has a fix to let it work with language variants such as
“en_GB-ize”. Thanks to Abdul-Lateef Haji-Ali for this fix!

There’s more code clean-up, build system updates, and a couple of null
pointer dereference fixes (thanks, GCC!).