[GHC] #13636: Enable -Wcpp-undef

[GHC]

#13636: Enable -Wcpp-undef
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 e5b3492f23c2296d0d8221e1787ee585331f726e tried (again) to enable `-Wcpp-
 undef` in `mk/warnings.mk`,
 {{{#!make
 SRC_HC_OPTS_STAGE1 += $(WERROR) #-Wcpp-undef
 SRC_HC_OPTS_STAGE2 += $(WERROR) #-Wcpp-undef
 }}}

 Unfortunately, this caused build failures in boot libraries on some
 platforms (e.g. `time` on Darwin). Resolve these and reenable `-Wcpp-
 undef`.

 An alternative would be to instead only add `-Wcpp-undef` to
 `GhcRtsHcOpts`, `GhcStage1HcOpts` and `GhcStage2HcOpts`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13636>
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