We use MACOSX_DEPLOYMENT_TARGET=10.10 in nixpkgs and some darwin
packages like CoreFoundation are based on the 10.10 sources from
opensource.apple.com.
This is the first time since 5.9 that we also update `qtwebkit`.
`qtwebkit` is not maintained by Qt anymore and thus, we switch to the
community port as for example arch has done. To prevent pulling in
single patches, we just stick to the latest git version.