Chromium build failure due to "gcc: Fix purity" [git-bisect]

[aristidb]

For me, and also for Hydra, chromium stopped building due to an error with "sysroot".

I used git-bisect (only took ~12h :D) to track down the error to the commit 7fe89be ("gcc: Fix purity").

The first failing Hydra build shows the same error message: http://hydra.nixos.org/build/13195919/nixlog/1

I suspect the relevant change is (in pkgs/build-support/gcc-wrapper/gcc-wrapper.sh):

+    NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE --sysroot=/var/empty"

git-bisect details:

$ git bisect bad master
$ git bisect good 8dcf70f9185b6e466aff067e504db7b0255e87f9
$ git bisect run nix-build . -A pkgs.chromium
[...]
7fe89be1464dd0263c7c987d2460613f44c7b229 is the first bad commit
commit 7fe89be1464dd0263c7c987d2460613f44c7b229
Author: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date:   Mon Jun 30 16:39:39 2014 +0200

    gcc: Fix purity

    Somewhere the no-sys-dirs.patch got disabled, so gcc was looking in
    /usr/local/include and /usr/lib. Since I can't fix the patch easily,
    I've borrowed the --sysroot trick from clang-wrapper. This causes
    builtin paths to be prefixed with /var/empty
    (e.g. /var/empty/usr/lib), which don't exist.

:040000 040000 ec9ea9c8b843f9a318783d41e23ebe7be8ef6765 1dbabefeecbb2d4ecbb5d4878080c4eff138b42e M        pkgs
bisect run success

@edolstra @aszlig