nixos/peerflix: fix enable option description
nixos: add chronos service
tree: don't rely on $CC being set in the stdenv build environment

Fixes https://github.com/NixOS/nixpkgs/issues/5214.
netcdf: enabled HDF5, DAP, UDUNITS, and shared library
cran-packages: restructured
cran-packages: renamed the files from camelCase to kebab-case
cran-packages: sets hydraPlatforms of rpud and CARramps to none since those packages require non-free NVidia drivers
cran-packages: makes packages overridable
netcdf: removed unused configure flag
cran-packages: uses common variable names, doCheck and installFlags
cran-packages: marks broken packages `broken` instead of removeing them
cran-packages: updated sources
cran-packages: combined packages definition files
cran-packages: combined packages definition files further
all-packages.nix: the default entry point to the R packages set is now pkgs/development/r-modules/default.nix
r-modules: fix reference to cran-packages.nix
R: add meta.platforms attribute
r-modules: always set meta.platforms when meta.hydraPlatforms is set, too

By default, we take both values from the R derivation.
r-selectspm: cannot be built because it depends on the broken package ecespa