a) For `nixos-rebuild` you can set
  { nixpkgs.config.allowBroken = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowBroken = true; }
to ~/.config/nixpkgs/config.nix.

strip is /nix/store/0pjsgkxz0rp5baycq5sp2s72lrr5q9sg-binutils-2.30/bin/strip
stripping (with command strip and flags -S) in /nix/store/9gj3wnd7y1l2xcr5fwlmzlz5picsx8km-ocaml4.06.1-re2-0.11.0/lib
patching script interpreter paths in /nix/store/9gj3wnd7y1l2xcr5fwlmzlz5picsx8km-ocaml4.06.1-re2-0.11.0
checking for references to /build in /nix/store/9gj3wnd7y1l2xcr5fwlmzlz5picsx8km-ocaml4.06.1-re2-0.11.0...
cannot build derivation '/nix/store/i34qml2yc1d6g14lbdkpx2p02la73cxg-ocaml4.06.1-core_extended-0.11.0.drv': 2 dependencies couldn't be built
cannot build derivation '/nix/store/8ixa39808asjy785wqkl019qmi853q22-ocaml4.06.1-async_shell-0.11.0.drv': 2 dependencies couldn't be built
cannot build derivation '/nix/store/7v3addrpyvqcd61g8rwc371jm9x3wwfn-ocaml4.06.1-core_bench-0.11.0.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/w54qv46n8sc5wb49j6x1dxmc5zwsv1fz-ocaml4.06.1-core_profiler-0.11.0.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/6x9hr4763w8lsi7wv48cny5y8m523llj-ocaml4.06.1-email_message-0.11.0.drv': 2 dependencies couldn't be built
error: build of '/nix/store/0whdsb0yi2aym2pdbi1j6k02xi9wjxhw-ocaml4.06.1-csvfields-0.11.0.drv', '/nix/store/5j9kykp772jjgm63lwb8x0a30fl5ni71-ocaml4.06.1-async_interactive-0.11.0.drv', '/nix/store/650j9lshryzkbnr4hlzsprcf7lpjp0zg-ocaml4.06.1-async_find-0.11.0.drv', '/nix/store/6dvsdpjqbmqc2qlr9nwm1x5h9xv1xgff-ocaml4.06.1-async_parallel-0.11.0.drv', '/nix/store/6qajdvdvkxvis9zzndwf2pbj9kzi1ka6-ocaml4.06.1-command_rpc-0.11.0.drv', '/nix/store/6x9hr4763w8lsi7wv48cny5y8m523llj-ocaml4.06.1-email_message-0.11.0.drv', '/nix/store/75c4mqhf3vk3n0g4raq9w5d2q54h7z8a-ocaml4.06.1-textutils-0.11.0.drv', '/nix/store/7v3addrpyvqcd61g8rwc371jm9x3wwfn-ocaml4.06.1-core_bench-0.11.0.drv', '/nix/store/82j0l19sawbv6b9z8xmw18396745rwhx-ocaml4.06.1-ecaml-0.11.0.drv', '/nix/store/8ixa39808asjy785wqkl019qmi853q22-ocaml4.06.1-async_shell-0.11.0.drv', '/nix/store/apf2q2aswnj9h2hj5xsjr337bxf7ri7s-ocaml4.06.1-incremental-0.11.0.drv', '/nix/store/awc210n0nnkpdy1pzbccvhjng6za6994-ocaml4.06.1-ocaml_plugin-0.11.0.drv', '/nix/store/h2lag1a2i8pimzimpcbi6kysijaif83y-ocaml4.06.1-expect_test_helpers-0.11.0.drv', '/nix/store/hcscizscq8203yw4ms2gdri4pbbf2k0h-ocaml4.06.1-async_unix-0.11.0.drv', '/nix/store/i34qml2yc1d6g14lbdkpx2p02la73cxg-ocaml4.06.1-core_extended-0.11.0.drv', '/nix/store/iaq3rv7w2viijiddvwf1w75lbkcpzfrb-ocaml4.06.1-core-0.11.1.drv', '/nix/store/n24rsniwp960i36b39f0l20gwqgxzw9j-ocaml4.06.1-async_ssl-0.11.0.drv', '/nix/store/r9fvs49da4bp5n811cg3dgmf79yvdmsi-ocaml4.06.1-async-0.11.0.drv', '/nix/store/viavhswpsj0mygprlhlqzhkxviz40fpf-ocaml4.06.1-rpc_parallel-0.11.0.drv', '/nix/store/w54qv46n8sc5wb49j6x1dxmc5zwsv1fz-ocaml4.06.1-core_profiler-0.11.0.drv', '/nix/store/yig7l0idcyrjmid9svy545sjkm8q39gh-ocaml4.06.1-async_extra-0.11.0.drv' failed

/nix/store/hzap1aj8wnrjh3n327i0fafj48i3bmhh-ocaml4.05.0-sexplib0-0.11.0
/nix/store/8hzc223q2hg57jv4w87vlbqrdskr2qrp-ocaml4.05.0-shexp-0.11.1
/nix/store/0cgxv5m3cr3bvz3103x3vxr211g0pq6b-ocaml4.05.0-spawn-0.12.0
/nix/store/bqnb1h1wvqkav9q79bxypsm4jxyvax11-ocaml4.05.0-splittable_random-0.11.0
/nix/store/haaalagi27f2i57wr4xl6hhl2gwc6yac-ocaml4.05.0-stdio-0.11.0
/nix/store/i1ax97m5yphw2sadknpyg5ciy4nnviz9-ocaml4.05.0-textutils-0.11.0
/nix/store/f578q1dw92ky049n6bdfkacikk05icsp-ocaml4.05.0-textutils_kernel-0.11.0
/nix/store/867wfbj1kzwz9b4nlaq6xhmp1q9blbpp-ocaml4.05.0-topological_sort-0.11.0
/nix/store/byln7wqjl06mrlwbyljg8i71589wp8hj-ocaml4.05.0-typerep-0.11.0
/nix/store/1pnfb80b0gvx58raiaw7md1lv5x4zp3f-ocaml4.05.0-variantslib-0.11.0