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Detect when doxygen, dot, and pdflatex are installed.  When they are, enable
documentation.  Add --enable-documentation and --disable-documentation to
override the autodetection.  Install the docs to the configured docdir (usually
$prefix/share/doc/libvdpau).

Steal the XORG_CHANGELOG macro from X.Org and stash it in acinclude.m4.