Improve automake handling of the docbook documentation.
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.


git-svn-id: 03f0f1727258f1959b7b485b30a676bafdb78148@5145 0101bb08-14d6-0310-b084-bc0e0c8e3800

libninjam/libvorbis,Distrotech/libvorbis,brion/vorbis,pcwalton/vorbis,OffByOneStudios/vorbis,ShiftMediaProject/vorbis,TitaniumEagle/libvorbis,wighawag/vorbis,KTXSoftware/vorbis,libninjam/libvorbis,Rillke/vorbis,TitaniumEagle/libvorbis,brion/vorbis,OffByOneStudios/vorbis,OffByOneStudios/vorbis,wighawag/vorbis,pcwalton/vorbis,TitaniumEagle/libvorbis,libninjam/libvorbis,libninjam/libvorbis,pcwalton/vorbis,ShiftMediaProject/vorbis,ShiftMediaProject/vorbis,KTXSoftware/vorbis,ShiftMediaProject/vorbis,Rillke/vorbis,brion/vorbis,jdm/vorbis,jdm/vorbis,jdm/vorbis,wighawag/vorbis,ShiftMediaProject/vorbis,Rillke/vorbis,brion/vorbis,jdm/vorbis,pcwalton/vorbis,Rillke/vorbis,Distrotech/libvorbis,TitaniumEagle/libvorbis,pcwalton/vorbis,OffByOneStudios/vorbis,wighawag/vorbis,KTXSoftware/vorbis,Distrotech/libvorbis,jdm/vorbis,Distrotech/libvorbis,ShiftMediaProject/vorbis,KTXSoftware/vorbis