message: Support override CLANG_RESOURCE_DIR
  The result from clang commandline is something like
  /usr/lib/llvm-7/lib/clang/7.0.1
  Which means I need to recompile when the clang is upgraded to newer 7.x.x
  However the system has a convenient symlink, which is
  /usr/include/clang/7.
diff: @@ -120,6 +120,7 @@ endif()
  # Find Clang resource directory with Clang executable.
  +if(NOT CLANG_RESOURCE_DIR)
  find_program(CLANG_EXECUTABLE clang)
  if(NOT CLANG_EXECUTABLE)
  message(FATAL_ERROR "clang executable not found.")
  @@ -137,6 +138,7 @@ if(CLANG_FIND_RESOURCE_DIR_RESULT)
  message(FATAL_ERROR "Error retrieving Clang resource directory with Clang \
  executable. Output:\n ${CLANG_FIND_RESOURCE_DIR_ERROR}")
  endif()
  +endif()
  set_property(SOURCE src/utils.cc APPEND PROPERTY COMPILE_DEFINITIONS
  CLANG_RESOURCE_DIRECTORY=R"\(${CLANG_RESOURCE_DIR}\)")
  