hdf5_ep

[svn-r9905] Purpose:
fix bugs.

Description:
The three variables, g_nTasks, outBuff, and outBuffOffset, were
declared in ph5diff.h.  That means they are declared in various
*.c files that include it, thus having the appearance of multiple
defined.  Most C compilers are forgiving of this error but
the g++ rejects this practice.

Solution:
Make it only an extern declaration in ph5diff.h but really
defined them in h5diff_util.c

Platforms tested:
sleipnir using g++ since this only failed in Daily test.
Also testes in heping, serial and parallel modes.