[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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Description changed by hvr:

@@ -13,1 +13,1 @@
- with out
+ with our `CONF_CC_OPTS_STAGE[012]` scheme...

New description:

 GCC 5.x started defaulting to C99 (or rather gnu99 mode), while Clang has
 been defaulting to C99 (or even C11) for a bit longer, as has Apple's
 patched GCC.

 So we should try to consistently turn on (if needed) C99 (or C11) support
 for all compilers supported by GHC, and adapt the code-base to expect C99
 compliance (which probably allows us to get rid of a couple of compat-CPP
 and make the C code more idiomatic and readable in the process...).


 GNU Autoconf provides a `AC_PROG_CC_C99` which figures out what's needed
 to have the compiler turn on C99 support, but we need to integrate that
 with our `CONF_CC_OPTS_STAGE[012]` scheme...

 Requiring C99 will effectively rule out GCC 3.x (which is old enough that
 we can drop support for that), and possibly also GCC < 4.3 at some point
 (for reference,  even Debian 6 Squeeze didn't have GCC 4.2 anymore), see
 also https://gcc.gnu.org/c99status.html

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11757#comment:1>
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