[svn-r9988] Purpose:

Bug fix

Description:
pmake (on modi4, for instance) complains about undefined variables
if it is run without the -V flag, which turns those errors to warnings.

Solution:
Added test to configure.in to see if $MAKE will allow Makefiles
with undefined variables.  If not, sets -V flag in AM_MAKEFLAGS.
Ensured that all custom make targets use AM_MAKEFLAGS.
Also defined all variables that caused errors in top-level Makefile.am.
This means that pmake can be used to build hdf5 *from the top level
only*.  To run make from a subdirectory, still need to use -V flag
(or use make or gmake).

Platforms tested:
modi4, heping, copper, sleipnir

config/commence.am

@@ -1,8 +1,6 @@
-#------------------------------------------------------------ -*- makefile -*-
-# The following section of this makefile comes from the
-# `./config/commence' file which was generated with config.status
-# from `./config/commence.in'.
-#-----------------------------------------------------------------------------
+## config/commence.am
+## Textually included in the beginning of every HDF5 Makefile.am
+## Contains definitions, etc. used across multiple Makefiles.
 
 # Shell commands used in Makefiles
 RM=rm -f