hdf5doc

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