Improve automake handling of the docbook documentation.
We no longer require automake 1.6 to build things, and 'make dist' will now fail if the documentation hasn't been built. Passing --enable-docs to autogen/configure is still necessary. 'make clean' now removes intermediate files and 'make maintainer-clean' removes the built docs themselves. svn path=/trunk/vorbis/; revision=5145
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