gnu: doxygen: Add propagated input graphviz.

* gnu/packages/doxygen.scm (doxygen): Add propagated input graphviz.

gnu: Add doxygen.

* gnu/packages/doxygen.scm: New file.
* gnu/packages/patches/doxygen-test.patch,
  gnu/packages/patches/doxygen-tmake.patch: New files.
* gnu-system.am (GNU_SYSTEM_MODULES, dist_patch_DATA): Register the new files.