CMake: Make sure to detect LibXml2 when Clang is built standalone

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170444 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f7099ac..0ad0557 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -66,6 +66,11 @@
   set( CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib )
 
   set( CLANG_BUILT_STANDALONE 1 )
+
+  find_package(LibXml2)
+  if (LIBXML2_FOUND)
+    set(CLANG_HAVE_LIBXML 1)
+  endif ()
 endif()
 
 set(CLANG_RESOURCE_DIR "" CACHE STRING