qt4: parallel-build impurity fix

Modified to avoid unnecessary rebuilds.
Merge branch 'master' into x-updates

* master: (118 commits)
  curl: Update to 7.30.0
  Revert "experimental/shadow"
  xxdiff: update to current HEAD of the source repository
  hatari: fix pthread_cancel+gcc_s thing, and some spacing
  Auto-update some packages; no early build failures (SBCL and Wine not yet fully built)
  Distrho: fix build: pin premake to version 3
  python-deform: unittest2 needed on python 26
  Revert "let hydra also build python26Packages"
  Add hatari Atari St emulator.
  magit: fix download url
  Fix indention, remove maintainers.
  php: Add 5.4
  libextractor: make gtk support work again
  Adding doodle.
  munge 0.5.10: New package, a credentials handler for clusters
  git: Properly use symlinks instead of hard links
  Revert "git: fix build on FreeBSD"
  haskell-hashable: update to version 1.2.x
  pkgs/top-level/release-haskell.nix: cosmetic
  haskell-haskeline: drop obsolete version 0.6.4.7
  ...

Conflicts:
	pkgs/servers/x11/xorg/overrides.nix