hdf5doc

Purpose:
Feature - conditional compilation

Description:
SRB file driver and tests are now compiled only when SRB is enabled
(using --with-srb during configure).

Solution:
Added an automake conditional in configure.in, altered Makefiles.am in
src and test directories to depend on that conditional.
This should make a nice example for posterity to add conditionally
compiled sources.

Platforms tested:
heping (only configure change)