Re: New cpp0 warning in 3.1 breaks configure (autoconf)

[H}kan Hjort <d95hjort at dtek dot chalmers dot se>]

A.k.a "changing search order for system directory"

Let me try to exemplify.  The following configure script will fail
if --with-breakme=/usr/include (or any other default system include
path, like /usr/local/include on most systems) is given.
This is new with GCC 3.1.  
To complicate things further BSD does not have /usr/local/include as a
default include directory so testing for that and excluding it will not
work.

--begin configure.in--
AC_INIT(configure.in)
AC_PROG_CC

dnl First one to force it to figure out that gcc -E is a good preprocessor
AC_CHECK_HEADER(stdlib.h)

saved_CPPFLAGS=$CPPFLAGS

AC_ARG_WITH(breakme, 
  [  --with-breakme=path   specify a location of the headers files to use ], 
  CPPFLAGS="$CPPFLAGS -I$withval")

AC_CHECK_HEADER(string.h,,[AC_MSG_ERROR([Didn't find string.h])])

CPPFLAGS=$saved_CPPFLAGS

AC_OUTPUT(Makefile)
--end configure.in--

Running this (after autoconf) as shown below ends with an error.

>$ ./configure --with-breakme=/usr/local/include
...
checking for string.h... no
configure: error: Some thing is seriously messed up

>$ tail -8 config.log 
configure:875: checking for string.h
configure:885: gcc3 -E  -I/usr/local/include conftest.c >/dev/null 2>conftest.out
cpp0: warning: changing search order for system directory "/usr/local/include"
cpp0: warning:   as it has already been specified as a non-system directory
configure: failed program was:
#line 880 "configure"
#include "confdefs.h"
#include <string.h>

Here is another message http://gcc.gnu.org/ml/gcc/2001-07/msg00470.html
from last year that takes up the problem with this 'default on' no way
to turn it off warning. (Not exatly the same problem but..)
And reports in that thread of extra failurs in the testsuite from this
change.

Further evidens of problems with these warnings can be found here
http://gcc.gnu.org/ml/gcc-bugs/2002-04/msg00149.html

Where can the forbidden directories be found?  
(Guessing based on gcc -v -E ..., /usr/local/include /usr/include)
 
Is there anyway to get rid of this warning?   Why is it on by default?

How are one supposed to workaround this in configure scripts?

-- 
Håkan Hjort
d95hjort@dtek.chalmers.se