Use opam to actually install eliom
Use propagatedBuildInputs in eliom
nixos: fix two pipefail problems

It failed since 3c6efec2c09, i.e. #4453.
Now it should "work" the same as before.
cairo: major update 1.12.* -> 1.14.0

ABI diff only suggest that one enum value got updated,
which should be fine for us.
http://upstream-tracker.org/compat_reports/cairo/1.12.16_to_1.14.0/abi_compat_report.html#Low_Risk_Problems
gtk2: bugfix update
glib: feature update 2.40.* -> 2.42.0

http://upstream-tracker.org/ looks good enough, and so do the NEWS.
mesa: bugfix update

I'm still having some problems with 10.3.1 compilation.
xorg: updates some modules; mostly minor changes
libdrm: update
libiconv: update to 1.14 (close #4419)
gstreamer-1: maintenance updates of all
libevdev: small update
p11-kit: do not use libiconv where not needed

Building libiconv even fails on Linux due to missing gets.
fontconfig: update 2.10.2 -> 2.11.1. Close #4410, #2050
fontconfig: add an upstream bugfix, remove other config inclusion
pango: big update; it was waiting on fontconfig-2.11
gst-1 video-related bumps (released yesterday)

pitivi looks fine, CC @iyzsong.
gtk3 stack libs: update all to gnome-3.14 branches

All NEWS look safe and relatively quiet.
Merge recent master into staging

Hydra nixpkgs: ?compare=1156478
libpng: bugfix update

This might fix the often-seen error messages.