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.

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

Not all of these actually show up in the output, but it's unnecessary to use
these in Markdown anyway.

This causes typesetting issues in some cases (because it is interpreted as the
end of a sentence), and it's probably more accessible to write out fully
anyway.