Older versions of Doxygen warn about these.  So, remove them to make the build
less noisy since they are all either unused or set to their defaults anyway.

These all have the default value anyway.

This allows the LV2 source distribution to be used as an include path for
compilers and an LV2_PATH for applications, at the expense of self-contained
bundles.  That's a nice idea, but it made LV2 itself weird and annoying to
depend on.  This rearranges things so that directories in the source tree
correspond more closely to installation directories.

To make this possible, the "aux" directory in the documentation output has been
changed to "style", to avoid the reserved name "aux" on Windows.

This has better compatibility with tools that manipulate the colour scheme,
like Dark Reader.