net-libs/neon doesn't work with >=net-libs/gnutls-2.11.1[nettle].
http://lists.manyfish.co.uk/pipermail/neon/2010-October/001283.html http://lists.manyfish.co.uk/pipermail/neon/2011-April/001351.html
Upstream seems not to care?
My patch has been applied by upstream. Fixed in net-libs/neon-0.29.6.