Revert "cabal: enable multi-threaded builds with GHC 7.8.x or later"

This likely exacerbates the non-determinism in ghc package ids, so until
that is fixed let's live with the slow builds.

This reverts commit 817c0e41443a5176baf6dd9b422878fdccecd266.
Merge pull request #4554 from shlevy/no-parallel-haskell

Revert "cabal: enable multi-threaded builds with GHC 7.8.x or later"
libpcap: pass --with-pcap=linux for all linux systems

...instead of just for i686-linux.

Without a --with-pcap= configure flag, the build system tries to
auto-detect the backend by poking in /dev, /usr/include etc... In the
pure nix world, this auto-detection fails. (Good, we don't want host
details to leak into the build.)

The end result of this (failed) auto-detection; no packet capture
possible:

  $ sudo tcpdump -i wlp2s0
  tcpdump: live packet capture not supported on this system

This fixes the above issue on 64-bit linux systems, and probably other
architectures supported by linux.
Merge branch 'master' of https://github.com/nixos/nixpkgs into me
wiki