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