``clang -Xclang <option> ...`` to force the driver pass ``<option>`` to
``clang -cc1``.
+I get errors about some headers being missing (``stddef.h``, ``stdard.h``)
+--------------------------------------------------------------------------
+
+Some header files (``stddef.h``, ``stdard.h``, and others) are shipped with
+Clang --- these are called builtin includes. Clang searches for them in a
+directory relative to the location of the ``clang`` binary. If you moved the
+``clang`` binary, you need to move the builtin headers, too.
+
+More information can be found in the :ref:`libtooling_builtin_includes`
+section.
+