Use Xcode relative compilers when possible

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@151108 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/testit b/test/testit
index d30e78c..774f96f 100755
--- a/test/testit
+++ b/test/testit
@@ -10,7 +10,12 @@
 
 if [ -z $CC ]
 then
-	CC=clang++
+	if which xcrun >/dev/null
+	then
+		CC="xcrun clang++"
+	else
+		CC=clang++
+	fi
 fi
 
 if [ -z "$OPTIONS" ]