Configure scripts: `checkOS`: Make a bit more robust

`mingw64` and `mingw32` are now both accepted for `OSMinGW32`. This
allows us to cope with configs/triples that we haven't normalized extra
being what GNU `config.sub` does.

m4/fptools_set_haskell_platform_vars.m4

@@ -82,7 +82,7 @@ AC_DEFUN([FPTOOLS_SET_HASKELL_PLATFORM_VARS_SHELL_FUNCTIONS],
         solaris2)
             test -z "[$]2" || eval "[$]2=OSSolaris2"
             ;;
-        mingw32|windows)
+        mingw32|mingw64|windows)
             test -z "[$]2" || eval "[$]2=OSMinGW32"
             ;;
         freebsd)

m4/ghc_convert_os.m4

@@ -22,7 +22,7 @@ AC_DEFUN([GHC_CONVERT_OS],[
       openbsd*)
         $3="openbsd"
         ;;
-      windows|mingw32)
+      windows|mingw32|mingw64)
         $3="mingw32"
         ;;
       # As far as I'm aware, none of these have relevant variants