[GHC] #13217: configure script uses different CFLAGS from the actual build

[GHC]

#13217: configure script uses different CFLAGS from the actual build
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           Reporter:  dobenour       |             Owner:
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Build System   |           Version:  8.1
           Keywords:  autoconf       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Building GHC
  Unknown/Multiple                   |  failed
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 `configure` fails to set `CFLAGS` to the value that will actually be used.
 This can result in checks giving wrong results.  I discovered this when
 working on Phab:D2996, where it led to a build failure using Clang.

 Note that `-Werror` cannot be used during most checks in `configure`, but
 is necessary for `FP_CHECK_FLAG`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13217>
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