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.

It is more or less meaningless to have an explicit type of rdfs:Resource, and
some tools don't like it, since everything (including classes and properties)
is a Resource.  Since this is an individual, owl:Thing is a better type.

This was obviously wrong.  I'm not sure how this happened, probably a
copy/paste mistake.

For consistency, and because clang-tidy can't figure out if a define is an
include guard unless it ends the file.