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* Uninstall the docs if they were installed.
* Remember to distribute Doxyfile and vdpau_data_flow.png in the
  tarball.
Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>

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).