Allow disabling libXML in Clang's CMake build

CMakeLists.txt

 cmake_minimum_required(VERSION 2.8.8)
 
 # FIXME: It may be removed when we use 2.8.12.
 if(CMAKE_VERSION VERSION_LESS 2.8.12)
   # Invalidate a couple of keywords.
   set(cmake_2_8_12_INTERFACE)
   set(cmake_2_8_12_PRIVATE)
 else()
   # Use ${cmake_2_8_12_KEYWORD} intead of KEYWORD in target_link_libraries().
   set(cmake_2_8_12_INTERFACE INTERFACE)
@@ skipping 149 matching lines @@
       set(LLVM_INCLUDE_TESTS OFF)
     endif()
   endif()
 
   set( CLANG_BUILT_STANDALONE 1 )
   set(BACKEND_PACKAGE_STRING "LLVM ${LLVM_PACKAGE_VERSION}")
 else()
   set(BACKEND_PACKAGE_STRING "${PACKAGE_STRING}")
 endif()
 
-find_package(LibXml2)
-if (LIBXML2_FOUND)
-  set(CLANG_HAVE_LIBXML 1)
+# @LOCALMOD-BEGIN
+option(CLANG_ENABLE_LIBXML "Enable libXML" ON)
+if (CLANG_ENABLE_LIBXML)
+  find_package(LibXml2)
+  if (LIBXML2_FOUND)
+    set(CLANG_HAVE_LIBXML 1)
+  endif()
 endif()
+# @LOCALMOD-END
 
 set(CLANG_RESOURCE_DIR "" CACHE STRING
   "Relative directory from the Clang binary to its resource files.")
 
 set(C_INCLUDE_DIRS "" CACHE STRING
   "Colon separated list of directories clang will search for headers.")
 
 set(GCC_INSTALL_PREFIX "" CACHE PATH "Directory where gcc is installed." )
 set(DEFAULT_SYSROOT "" CACHE PATH
   "Default <path> to all compiler invocations for --sysroot=<path>." )
@@ skipping 342 matching lines @@
     ${CLANG_BINARY_DIR}/share/clang/cmake/ClangTargets.cmake
     DESTINATION share/clang/cmake)
 
   # Also copy ClangConfig.cmake to the build directory so that dependent projects
   # can build against a build directory of Clang more easily.
   configure_file(
     ${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules/ClangConfig.cmake
     ${CLANG_BINARY_DIR}/share/clang/cmake/ClangConfig.cmake
     COPYONLY)
 endif ()