[GHC] #16368: Boot GHC's libffi installation path leaks into stage1 build

[GHC]

#16368: Boot GHC's libffi installation path leaks into stage1 build
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        Reporter:  sgraf             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  ⊥
       Component:  Build System      |              Version:  8.6.3
  (Hadrian)                          |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by sgraf):

 Nix stores headers in /nix/store/xxx-libffi-3.2.1-dev/include, so it
 separates build artifacts from headers. So the problem is with the wrong
 setting
 [https://github.com/NixOS/nixpkgs/commit/a7c774300b7eab779afb9a81b3f02f3bed69c943
 #diff-50972848d68a2300257f35df16c0fd3aR156 here].

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16368#comment:4>
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