[libcxx][CMake] Move the warning to HandleOutOfTreeLLVM

[phosek]

Summary

This currently gives a warning when building libcxx under runtimes. When building as part of runtimes, the value of CMAKE_SOURCE_DIR is different from CMAKE_CURRENT_SOURCE_DIR, but runtimes build sets ${runtime}_STANDALONE_BUILD to 1 which triggers this warning. This was introduced by r286789.

[phosek]

On a related note, there seems to be difference in handling this part of configuration between libcxxabi and libunwind, and libcxx. For example, when libcxxabi or libunwind cannot find llvm-config they throw FATAL_ERROR while libcxx only prints WARNING and carries on. Would it make sense to unify this logic, and if so which is correct?

libcxx/trunk/CMakeLists.txt

if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   set(PACKAGE_VERSION 4.0.0svn)
   set(PACKAGE_STRING "${PACKAGE_NAME} ${PACKAGE_VERSION}")
   set(PACKAGE_BUGREPORT "[e-mail]")
 
   # Find the LLVM sources and simulate LLVM CMake options.
   include(HandleOutOfTreeLLVM)
 endif()
 
-if (LIBCXX_STANDALONE_BUILD AND NOT LLVM_FOUND)
-  message(WARNING "UNSUPPORTED LIBCXX CONFIGURATION DETECTED: "
-                  "llvm-config not found and LLVM_PATH not defined.\n"
-                  "Reconfigure with -DLLVM_CONFIG_PATH=path/to/llvm-config "
-                  "or -DLLVM_PATH=path/to/llvm-source-root.")
-endif()
-
 # Require out of source build.
 include(MacroEnsureOutOfSourceBuild)
 MACRO_ENSURE_OUT_OF_SOURCE_BUILD(
  "${PROJECT_NAME} requires an out of source build. Please create a separate
  build directory and run 'cmake /path/to/${PROJECT_NAME} [options]' there."
  )
 
 #===============================================================================

libcxx/trunk/cmake/Modules/HandleOutOfTreeLLVM.cmake

elseif(LLVM_CONFIG_PATH)
     list(GET CONFIG_OUTPUT 2 MAIN_SRC_DIR)
 
     set(LLVM_INCLUDE_DIR ${INCLUDE_DIR} CACHE PATH "Path to llvm/include")
     set(LLVM_BINARY_DIR ${LLVM_OBJ_ROOT} CACHE PATH "Path to LLVM build tree")
     set(LLVM_MAIN_SRC_DIR ${MAIN_SRC_DIR} CACHE PATH "Path to LLVM source tree")
     set(LLVM_CMAKE_PATH "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
   else()
     set(LLVM_FOUND OFF)
+    message(WARNING "UNSUPPORTED LIBCXX CONFIGURATION DETECTED: "
+                    "llvm-config not found and LLVM_PATH not defined.\n"
+                    "Reconfigure with -DLLVM_CONFIG_PATH=path/to/llvm-config "
+                    "or -DLLVM_PATH=path/to/llvm-source-root.")
     return()
   endif()
 
   if (EXISTS "${LLVM_CMAKE_PATH}")
     list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")
   elseif (EXISTS "${LLVM_MAIN_SRC_DIR}/cmake/modules")
     list(APPEND CMAKE_MODULE_PATH "${LLVM_MAIN_SRC_DIR}/cmake/modules")
   else()