hdf5_ep

[svn-r916] Problem:
In machine (e.g. O2K) where long long and long are the same sizes, it
ended up defining GB8LL as 1 which is way too small for the dataset size.

Solution:
Add an entry to define GB8LL if long can hold it.  When all fails,
define GB8LL as 0.  During runtime, enough_room() will detect this
special value and skips the test.

Platform tested:
O2K IRIX64.