hdf5_msb

[svn-r12342] Purpose:
bug fix.

Description:
Previous H5private.h would attempt to use the off64_t/stat64/etc
by checking just sizeof __INT64 being 8.  This caused errors in
some machines like alpha cluster which is a 64bit machine.  Its
compiler supports the type __int64 but it does not support
off64_t since its off_t is already 64bits big.  The above code
casued the compiler to complain about the unknown off64_t and
such.

Solution:
H5private.h:
Changed the code to use the pseudo standard of off64_t/stat64/etc
only if sizeof(off_t) is not 64 bits AND sizeof(off64_t) exists
and is 64 bits.  This assumes if off64_t is defined, all those
stat64, fstat64, etc are supported too.

configure.in:
configure:
Added the testing of sizeof(off64_t) to support the above solution.

Platforms tested:
h5committested.  Also tested at LANL QSC.