Build Haddocks with source hyperlinks unless doHscolour is false
chicken: Bump to 4.9.0.
texlive: enable building xdv2pdf and xdvipdfmx on darwin
texlive: add note to clarify why dvisvgm fails to work on darwin
pythonPackages.libcloud: update to 0.14.1
gnutls: security + maintenance update for both branches

Fixes #2813 CVE-2014-3466. No API/ABI changes in the updates.

Tests start to fail for gnutls31, so they get disabled,
but the 3.1 branch is already unused on master.
Merge pull request #2767 from jwiegley/hscolour

Build Haddocks with source hyperlinks unless doHscolour is false