fix typo

Thanks to "Gabor Greif" <ggreif@gmail.com> for reporting this problem.

The configure flag should be --with-default-sysroot as documented, and
not --with-sysroot.  The reason we don't want to define --with-sysroot
is that GCC has a configure flag by that name and it has a different
semantics.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@155844 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/configure b/configure
index 50db26c..f72dca4 100755
--- a/configure
+++ b/configure
@@ -5584,9 +5584,9 @@
 
 
 
-# Check whether --with-sysroot was given.
-if test "${with_sysroot+set}" = set; then
-  withval=$with_sysroot;
+# Check whether --with-default-sysroot was given.
+if test "${with_default_sysroot+set}" = set; then
+  withval=$with_default_sysroot;
 else
   withval=""
 fi