On Prefix, we have specified SYSROOT for llvm at build time.  But we
  do not need sysroot at runtime as we are not crosscompiling.

This patch disable the sysroot feature for cling, so that ACLiC can
work on Prefix.

Index: root-6.12.06/interpreter/cling/lib/Utils/Paths.cpp
===================================================================
--- root-6.12.06.orig/interpreter/cling/lib/Utils/Paths.cpp
+++ root-6.12.06/interpreter/cling/lib/Utils/Paths.cpp
@@ -57,11 +57,6 @@ using namespace clang;
 void CopyIncludePaths(const clang::HeaderSearchOptions& Opts,
                       llvm::SmallVectorImpl<std::string>& incpaths,
                       bool withSystem, bool withFlags) {
-  if (withFlags && Opts.Sysroot != "/") {
-    incpaths.push_back("-isysroot");
-    incpaths.push_back(Opts.Sysroot);
-  }
-
   /// User specified include entries.
   for (unsigned i = 0, e = Opts.UserEntries.size(); i != e; ++i) {
     const HeaderSearchOptions::Entry &E = Opts.UserEntries[i];