This doesn't work because lv2_check_syntax.py uses serdi, and so would need to
call it with a wrapper.  That's possible (and is how the test scripts work),
but this check doesn't need to run everywhere, so just disable it in cross
builds to avoid the issue.

Older versions of these tools can cause issues, but they are only useful for
development anyway.  So, just disable them by default to avoid build issues for
users on ancient systems.

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.