Backport r32353 from stdenv-udpates

* Get rid of Qt 4.6.  It doesn't compile with GCC 4.6.  There was only
  one application still using it (Caneda), but upgrading it allowed it
  to build with Qt 4.7.

Eelco, could you please fix the date in caneda version?

svn path=/nixpkgs/trunk/; revision=32359