validate failure on Mac OSX for ghc head (validate fails with clang)

[Carter Schonwald]

hey All, the current settings for warnings etc for ghc validate do not work
on mac,

this is because some gcc flag isn't in clang, or vice versa, or some
version of gcc related matter

the relevant error is:

error: unknown warning option '-Werror=unused-but-set-variable'; did you
mean '-Werror=unused-const-variable'? [-Werror,-Wunknown-warning-option]

the culprit seems to be apple clang, at least if my reading of the config
output by validate is correct

Configure completed successfully.

   Building GHC version  : 8.5.20171223
          Git commit id  : d7d0aa316f9d23d73ae617e0cc1b147907667ef4

   Build platform        : x86_64-apple-darwin
   Host platform         : x86_64-apple-darwin
   Target platform       : x86_64-apple-darwin

   Bootstrapping using   : /Users/carter/.install-ghc/ghc-8.2.2-hq/bin/ghc
      which is version   : 8.2.2

   Using (for bootstrapping) : /usr/bin/clang
   Using gcc                 : /usr/local/bin/gcc-7
      which is version       : 7.2.0
   Building a cross compiler : NO
   Unregisterised            : NO
   hs-cpp       : /usr/local/bin/gcc-7
   hs-cpp-flags : -E -undef -traditional


there seem to be several take aways:

1) clang doesn't support unused-but-set-variable
1.5) no one has run validate on OSX in the past few months with a vanilla
checkout / env?
2) how do i change the compiler selected for boot strapping, whether via
env variables or other twiddles

thanks!

-Carter
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