standalone build

        +-----------------------------------------------+
        | cmake                                         |
        |   CMakeLists.txt                              |
        |   $PREFIX/share/cmake/xo_macros/xo_cxx.cmake  |
        +-----------------------------------------------+
           |
           |                                                  +----------------------+
           +------------------------------------------------->| .build/docs/Doxyfile |
           |                                                  +----------------------+
           |                                                     ^
           |                                 (cmake              |
           |                                        /------------/
           |                                        |
           |     +---------------------------------------+    +-----------------+
           +---->| doxygen                               |--->| .build/docs/dox |
           |     |   $PREFIX/share/xo-macros/Doxyfile.in |    |   +- html/      |
           |     +---------------------------------------+    |   +- xml/       |
           |                                                  +-----------------+
           |                                                     |
           |                                        /------------/
           |                                        |
           |                                        v
           |     +---------------------------------------+    +--------------------+
           \---->| sphinx                                |--->| .build/docs/sphinx |
                 |   +- conf.py                          |    |   +- html/         |
                 |   +- _static/                         |    +--------------------+
                 |   +- *.rst                            |
                 +---------------------------------------+

umbrella build relies on top-level cmake macros

files

   README            this file
   CMakeLists.txt    build entry point
   conf.py           sphinx config
   _static           static files for sphinx

   index.rst         toplevel sphinx document; entry point