hdf5doc

Purpose:
Feature: tests now use parallel make and only run once

Description:
When make is invoked in parallel (using the -j option), several tests will
now be run simultaneously.  This should speed up testing on a number of
systems.
When a test passes, it creates a foo.chkexe or foo.chksh file, which prevents
the test from running again unless the test or library changes.

Solution:
Most of the changes live in config/conclude.am.  Added *.chkexe and *.chksh
files to the list of files to be cleaned at "make mostlyclean" time.
Parallel tests still run one at a time, but use the same make instructions
as serial tests.

Platforms tested:
mir, eirene, sleipnir, pommier, copper, modi4 (some errors occurred, probably
due to the test being run before updating hdf5.  Will re-run tests after
checkin.)