[GHC] #11757: Turn on (and require) C99/C11 mode by default

[GHC]

#11757: Turn on (and require) C99/C11 mode by default
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        Reporter:  hvr               |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Build System      |              Version:
      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 Herbert Valerio Riedel <hvr@…>):

 In [changeset:"afc48f8939b99a1a72b43b3e342d56193ed1f34c/ghc" afc48f8/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="afc48f8939b99a1a72b43b3e342d56193ed1f34c"
 Autoconf: detect and set CFLAGS/CPPFLAGS needed for C99 mode

 This is the first phase of addressing #11757 which aims to make C99
 support a base-line requirement for GHC and clean up the code-base to
 use C99 facilities when sensible.

 This patch exploits the logic/heuristic used by `AC_PROG_CC_C99` to
 determine the flags needed in case the C compiler isn't able to compile
 C99 code in its current mode. We can't use `AC_PROG_CC_C99` directly
 though because GHC's build-system expects CC to contain a filename
 without any flags, while `AC_PROG_CC_C99` would e.g. result in
 `CC="gcc -std=gnu99"`. Morever, we support different `CC`s for
 stage0/1/2, so we need a version of `AC_PROG_CC_C99` for which we can
 specify the `CC`/`CFLAGS` variables to operate on. This is what
 `FP_SET_CFLAGS_C99` does.

 Note that Clang has been defaulting to C99+ for a long time, while GCC 5
 defaults to C99+ as well. So this has mostly an affect on older GCCs
 versions prior to 5.0 and possibly compilers other than GCC/Clang (which
 are not officially supported for building GHC anyway).

 Reviewers: kgardas, erikd, bgamari, austin

 Reviewed By: erikd

 Differential Revision: https://phabricator.haskell.org/D2045
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11757#comment:2>
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