[compiler-rt] [CMake] Allow building builtins standalone out of tree without any llvm-config available

[mstorsjo]

Summary

This is the same as libcxxabi/libcxx do.

[phosek]

LGTM

cmake/Modules/CompilerRTUtils.cmake

macro(detect_target_arch)
   endif()
 endmacro()
 
 macro(load_llvm_config)
   if (NOT LLVM_CONFIG_PATH)
     find_program(LLVM_CONFIG_PATH "llvm-config"
                  DOC "Path to llvm-config binary")
     if (NOT LLVM_CONFIG_PATH)
-      message(FATAL_ERROR "llvm-config not found: specify LLVM_CONFIG_PATH")
+      message(WARNING "UNSUPPORTED COMPILER-RT CONFIGURATION DETECTED: "
+                      "llvm-config not found.\n"
+                      "Reconfigure with -DLLVM_CONFIG_PATH=path/to/llvm-config.")
     endif()
   endif()
+  if (LLVM_CONFIG_PATH)
     execute_process(
       COMMAND ${LLVM_CONFIG_PATH} "--obj-root" "--bindir" "--libdir" "--src-root"
       RESULT_VARIABLE HAD_ERROR
       OUTPUT_VARIABLE CONFIG_OUTPUT)
     if (HAD_ERROR)
       message(FATAL_ERROR "llvm-config failed with status ${HAD_ERROR}")
     endif()
     string(REGEX REPLACE "[ \t]*[\r\n]+[ \t]*" ";" CONFIG_OUTPUT ${CONFIG_OUTPUT})
     list(GET CONFIG_OUTPUT 0 BINARY_DIR)
     list(GET CONFIG_OUTPUT 1 TOOLS_BINARY_DIR)
     list(GET CONFIG_OUTPUT 2 LIBRARY_DIR)
     list(GET CONFIG_OUTPUT 3 MAIN_SRC_DIR)
 
     set(LLVM_BINARY_DIR ${BINARY_DIR} CACHE PATH "Path to LLVM build tree")
     set(LLVM_TOOLS_BINARY_DIR ${TOOLS_BINARY_DIR} CACHE PATH "Path to llvm/bin")
     set(LLVM_LIBRARY_DIR ${LIBRARY_DIR} CACHE PATH "Path to llvm/lib")
     set(LLVM_MAIN_SRC_DIR ${MAIN_SRC_DIR} CACHE PATH "Path to LLVM source tree")
 
     # Make use of LLVM CMake modules.
     # --cmakedir is supported since llvm r291218 (4.0 release)
     execute_process(
       COMMAND ${LLVM_CONFIG_PATH} --cmakedir
       RESULT_VARIABLE HAD_ERROR
       OUTPUT_VARIABLE CONFIG_OUTPUT)
     if(NOT HAD_ERROR)
       string(STRIP "${CONFIG_OUTPUT}" LLVM_CMAKE_PATH_FROM_LLVM_CONFIG)
       file(TO_CMAKE_PATH ${LLVM_CMAKE_PATH_FROM_LLVM_CONFIG} LLVM_CMAKE_PATH)
     else()
       file(TO_CMAKE_PATH ${LLVM_BINARY_DIR} LLVM_BINARY_DIR_CMAKE_STYLE)
       set(LLVM_CMAKE_PATH "${LLVM_BINARY_DIR_CMAKE_STYLE}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
     endif()
 
     list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")
     # Get some LLVM variables from LLVMConfig.
     include("${LLVM_CMAKE_PATH}/LLVMConfig.cmake")
 
     set(LLVM_LIBRARY_OUTPUT_INTDIR
       ${LLVM_BINARY_DIR}/${CMAKE_CFG_INTDIR}/lib${LLVM_LIBDIR_SUFFIX})
+  endif()
 endmacro()
 
 macro(construct_compiler_rt_default_triple)
   if(COMPILER_RT_DEFAULT_TARGET_ONLY)
     if(DEFINED COMPILER_RT_DEFAULT_TARGET_TRIPLE)
       message(FATAL_ERROR "COMPILER_RT_DEFAULT_TARGET_TRIPLE isn't supported when building for default target only")
     endif()
     set(COMPILER_RT_DEFAULT_TARGET_TRIPLE ${CMAKE_C_COMPILER_TARGET})