add configure flag --with-default-sysroot

llvm-svn: 154791
diff --git a/llvm/autoconf/configure.ac b/llvm/autoconf/configure.ac
index 0a2c8b6..a3961ef 100644
--- a/llvm/autoconf/configure.ac
+++ b/llvm/autoconf/configure.ac
@@ -838,6 +838,13 @@
 AC_DEFINE_UNQUOTED(GCC_INSTALL_PREFIX,"$withval",
                    [Directory where gcc is installed.])
 
+AC_ARG_WITH(sysroot,
+  AS_HELP_STRING([--with-default-sysroot],
+    [Add --sysroot=<path> to all compiler invocations.]),,
+    withval="")
+AC_DEFINE_UNQUOTED(DEFAULT_SYSROOT,"$withval",
+                   [Default <path> to all compiler invocations for --sysroot=<path>.])
+
 dnl Allow linking of LLVM with GPLv3 binutils code.
 AC_ARG_WITH(binutils-include,
   AS_HELP_STRING([--with-binutils-include],