testing sizeof MPI_Offset

[Robert Latham]

As a result of the large file support added in the recent release, it
is more important that MPI_Offset is big enough.  Right now we assume
the underlying MPI implementation uses an 8 byte type to represent
MPI_Offset.  It would be ideal to have a configure-time test that
checks the size of MPI_Offset and exits early if it is not large
enough.

With MPICH, this is not too hard to do with autoconf macros.  However,
on platforms like the SP and the NEC machines, I'm not sure how to
compile a test program that both understands 'MPI_Offset' and can run
w/in the context of a configure script.  

Does anyone here have suggestions how to test for this possible
problem in a portable way?  

In the worst case, we can check this at runtime and exit, but that's
less than ideal.

Thanks
==rob

-- 
Rob Latham
Mathematics and Computer Science Division    A215 0178 EA2D B059 8CDF
Argonne National Labs, IL USA                B29D F333 664A 4280 315B