diff --git a/tools/c-index-test/CMakeLists.txt b/tools/c-index-test/CMakeLists.txt
index d90dc6d..d850411 100644
--- a/tools/c-index-test/CMakeLists.txt
+++ b/tools/c-index-test/CMakeLists.txt
@@ -24,6 +24,6 @@
 
 # If libxml2 is available, make it available for c-index-test.
 if (CLANG_HAVE_LIBXML)
-  include_directories(${LIBXML2_INCLUDE_DIR})
+  include_directories(SYSTEM ${LIBXML2_INCLUDE_DIR})
   target_link_libraries(c-index-test ${LIBXML2_LIBRARIES})
 endif()