CMake: Turn LLVM_ENABLE_LIBXML2 into a tri-state option

In addition to the current ON and OFF options, this adds the FORCE_ON
option, which causes a configuration error if libxml2 cannot be used.

Differential revision: https://reviews.llvm.org/D40050

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@318209 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -347,7 +347,7 @@ set(LLVM_TARGET_ARCH "host"
 
 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)
 

cmake/config-ix.cmake

@@ -170,6 +170,10 @@ if( NOT PURE_WINDOWS AND NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
   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)