OS X: disable bootstrapping info for 'ar'

Xcode 4.3 installs the command line tools in a different location as earlier versions of Xcode. With the bootstrapping info, the build fails if the bootstrap compiler was built with an older version of Xcode.

configure.ac

@@ -131,10 +131,17 @@ if test "$WithGhc" != ""; then
   GhcMinVersion2=`echo "$GhcMinVersion" | sed 's/^\\(.\\)$/0\\1/'`
   GhcCanonVersion="$GhcMajVersion$GhcMinVersion2"
 
+  BOOTSTRAPPING_GHC_INFO_FIELD([OS_STAGE0],[target os],['unkown'])
   BOOTSTRAPPING_GHC_INFO_FIELD([CC_STAGE0],[C compiler command],['$(CC)'])
-  BOOTSTRAPPING_GHC_INFO_FIELD([AR_STAGE0],[ar command],['$(AR)'])
-  BOOTSTRAPPING_GHC_INFO_FIELD([AR_OPTS_STAGE0],[ar flags],['$(AR_OPTS)'])
-  BOOTSTRAPPING_GHC_INFO_FIELD([ArSupportsAtFile_STAGE0],[ar supports at file],['$(ArSupportsAtFile)'])
+  if test "x$OS_STAGE0" != "xOSDarwin"; then
+    BOOTSTRAPPING_GHC_INFO_FIELD([AR_STAGE0],[ar command],['$(AR)'])
+    BOOTSTRAPPING_GHC_INFO_FIELD([AR_OPTS_STAGE0],[ar flags],['$(AR_OPTS)'])
+    BOOTSTRAPPING_GHC_INFO_FIELD([ArSupportsAtFile_STAGE0],[ar supports at file],['$(ArSupportsAtFile)'])
+  else
+    AR_STAGE0='$(AR)'
+    AR_OPTS_STAGE0='$(AR_OPTS)'
+    ArSupportsAtFile_STAGE0='$(ArSupportsAtFile)'
+  fi
 fi
 
 dnl ** Must have GHC to build GHC, unless --enable-hc-boot is on