[GHC] #11684: Fix tests with Clang

[GHC]

#11684: Fix tests with Clang
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        Reporter:  erikd             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by erikd):

 If I hack `aclocal.m4` to add `-Qunused-arguments` so that it end up in
 the `settings` file then do `./configure --with-gcc=clang` and build, the
 the stage0 fails building `ghc-cabal` because GCC doesn't understand
 `-Qunused-arguments`.

 @thomie should the stage0 C compiler be using the CFLAGs from the
 `settings` file?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11684#comment:2>
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