Autoconf error on NetBSD

[Peter Cooper]

Further to Riccardo's email about strange things in the configure
stage on NetBSD, we get output like:

checking grp.h presence... yes
checking for grp.h... yes
checking sys/mount.h usability... no
checking sys/mount.h presence... yes
configure: WARNING: sys/mount.h: present but cannot be compiled
configure: WARNING: sys/mount.h: check for missing prerequisite headers?
configure: WARNING: sys/mount.h: proceeding with the preprocessor's result
configure: WARNING:     ## ------------------------------------ ##
configure: WARNING:     ## Report this to bug-autoconf@gnu.org. ##
configure: WARNING:     ## ------------------------------------ ##
checking for sys/mount.h... yes
checking for sys/types.h... (cached) yes

I reported the error to bug-autoconf and received the following reply from
Akim Demaille <akim@epita.fr>

> Please, report this to the package maintainers, as it's a configure.ac
> problem.  The Autoconf 2.57 doc says:
> 
>       Previous versions of Autoconf merely checked whether the header was
>    accepted by the preprocessor.  This was changed because the old test was
>    inappropriate for typical uses.  Headers are typically used to compile,
>    not merely to preprocess, and the old behavior sometimes accepted
>    headers that clashed at compile-time.  If you need to check whether a
>    header is preprocessable, you can use `AC_PREPROC_IFELSE' (*note
>    Running the Preprocessor::).
>    
>       This scheme, which improves the robustness of the test, also requires
>    that you make sure that headers that must be included before the
>    HEADER-FILE be part of the INCLUDES, (*note Default Includes::).  If
>    looking for `bar.h', which requires that `foo.h' be included before if
>    it exists, we suggest the following scheme:
>    
>    
>    AC_CHECK_HEADERS([foo.h])
>    AC_CHECK_HEADERS([bar.h], [], [],
>    [#if HAVE_FOO_H
>    # include <foo.h>
>    # endif
>    ])
> 
> Thanks!

Perhaps an autoconf guru might be interested in looking at this?
I had a peek around, but nothing totally obvious seemed to be the 
real cause.

Peter