-@samp{restrict} to be the empty string on a pre-C99 host, the
-definition is consistent for all include files.
-
-You should include Gnulib-provided headers before system headers,
-so that Gnulib-provided headers can adjust how a system header
-behaves.
+@samp{restrict} to be the empty string on a pre-C99 host, or a macro
+like @samp{_FILE_OFFSET_BITS} that affects the layout of data
+structures, the definition is consistent for all include files.
+Also, on some platforms macros like @samp{_FILE_OFFSET_BITS} and
+@samp{_GNU_SOURCE} may be ineffective, or may have only a limited
+effect, if defined after the first system header file is included.