[v3] doxygen updates

[Benjamin Kosnik]

>> INPUT      = \
>[...]
>>            include/i686-pc-linux-gnu/bits
>
>This seems wrong.  Granted that few if any people will be generating the docs
>from scratch on any other build system :-),  but this still seems inflexible.

Right.

This really needs host_alias substitutions.

>>   * docs/doxygen/filter: Remove.
>>   * docs/doxygen/filter.sed: Remove.
>
>Why?  This is what translated the uglified names in the source to more
>readable names (without abbreviations and tons of underscores) in the output.
>I looked for some other part that was now doing this, but can't find it.

The preprocessing support in doxygen itself is now better. We haven't
been using these filters for a while now.

-benjamin