[svn-r5666] Work around Darwin strict behavior and sys/stat.h in ROMIO.
When MPICH2 is configured with --enable-strict it has the effect of removing the u_char, u_short, u_int, and u_long types from sys/types.h. Unfortunately, sys/stat.h still uses these types whether or not they are defined. So we define them ourselves if they are not present so that our checks involving sys/stat.h will succeed later. Reviewed by balaji@.