Index: llvm/trunk/CMakeLists.txt
===================================================================
--- llvm/trunk/CMakeLists.txt
+++ llvm/trunk/CMakeLists.txt
@@ -347,7 +347,7 @@
 
 option(LLVM_ENABLE_TERMINFO "Use terminfo database if available." ON)
 
-option(LLVM_ENABLE_LIBXML2 "Use libxml2 if available." ON)
+set(LLVM_ENABLE_LIBXML2 "ON" CACHE STRING "Use libxml2 if available. Can be ON, OFF, or FORCE_ON")
 
 option(LLVM_ENABLE_LIBEDIT "Use libedit if available." ON)
 
Index: llvm/trunk/cmake/config-ix.cmake
===================================================================
--- llvm/trunk/cmake/config-ix.cmake
+++ llvm/trunk/cmake/config-ix.cmake
@@ -170,6 +170,10 @@
   endif()
 endif()
 
+if (LLVM_ENABLE_LIBXML2 STREQUAL "FORCE_ON" AND NOT LLVM_LIBXML2_ENABLED)
+  message(FATAL_ERROR "Failed to congifure libxml2")
+endif()
+
 check_library_exists(xar xar_open "" HAVE_LIBXAR)
 if(HAVE_LIBXAR)
   set(XAR_LIB xar)