Add purity to GHC builds on Darwin.

pkgs/development/haskell-modules/gcc-clang-wrapper.sh

@@ -0,0 +1,46 @@
+#!/bin/sh
+
+inPreprocessorMode () {
+    hasE=0
+    hasU=0
+    hasT=0
+    for arg in "$@"
+    do
+        if [ 'x-E' = "x$arg" ];             then hasE=1; fi
+        if [ 'x-undef' = "x$arg" ];         then hasU=1; fi
+        if [ 'x-traditional' = "x$arg" ];   then hasT=1; fi
+    done
+    [ "$hasE$hasU$hasT" = '111' ]
+}
+
+extraClangArgs="-Wno-invalid-pp-token -Wno-unicode -Wno-trigraphs"
+
+adjustPreprocessorLanguage () {
+    newArgs=''
+    while [ $# -gt 0 ]
+    do
+        newArgs="$newArgs $1"
+        if [ "$1" = '-x' ]
+        then
+            shift
+            if [ $# -gt 0 ]
+            then
+                if [ "$1" = 'c' ]
+                then
+                    newArgs="$newArgs assembler-with-cpp"
+                else
+                    newArgs="$newArgs $1"
+                fi
+            fi
+        fi
+        shift
+    done
+    echo $newArgs
+}
+
+if inPreprocessorMode "$@"
+then
+    exec clang $extraClangArgs `adjustPreprocessorLanguage "$@"`
+else
+    exec clang $extraClangArgs "${@/-nodefaultlibs/}"
+fi