configure wrongly caches results

[Patrice Dumas]

Hi,

I have different libraries that implements te same interface (the netcdf
interface) and each one comes with a netcdf.h file in a specific directory. 
I would like to check headers against each of these libraries in turn. So I 
set CPPFLAGS to each of the include directories in turn. However 
configure doesn't retry the test when CPPFLAGS changes and instead uses the 
cached value from previous test.

Here is a simplified  example of the tests:

CPPFLAGS_save="$CPPFLAGS"
CPPFLAGS="$CPPFLAGS -I/usr/include/hdf2"
AC_CHECK_HEADERS([netcdf.h],[echo "found with hdf"])
CPPFLAGS="$CPPFLAGS_save"

CPPFLAGS_save="$CPPFLAGS"
CPPFLAGS="$CPPFLAGS -I/usr/include/libnc-dap/"
AC_CHECK_HEADERS([netcdf.h],[echo "found with libnc-dap"])
CPPFLAGS="$CPPFLAGS_save"


If the first test fails, the second also fails, even if netcdf.h is in 
/usr/include/libnc-dap/. The output is in that case

checking for netcdf.h... no
checking for netcdf.h... (cached) no

If I comment out the first test, the second test succeed.

So it seems that there is a kind of caching that cannot be disabled and
that is preventing such tests to work. I have read a bit of documentation 
but haven't found how to disable that caching. Indeed what I read in
(autoconf) Cache Files is:

"   By default, `configure' uses no cache file (technically, it uses
`--cache-file=/dev/null'), to avoid problems caused by accidental use
of stale cache files."

Any thought on that question?

With autoconf 2.59.

--
Pat