libublio uses BSD style data structure macros via queue.h and tree.h,
something which is present in a form in all four major BSD variants.

Upon trying to compile libublio on xBSD, one may ask: how do libublio's
{quee,tree}.h headers relate to those of xBSD, and is it possible to use
the native xBSD versions of these header files indeed?

Briefly: you can use the native queue.h on all BSDs but you can use the
native tree.h only on FreeBSD.

In details:

- queue.h is taken from OpenBSD as is. I choose the OpenBSD version because
  that seemed to be the least stuffed with system-specific stuff. However,
  those macros of queue.h which I use do the same on all BSDs.

- tree.h was taken from FreeBSD with some modifications. I choose the FreeBSD
  version for two features:
  - it can generate a static tree function kit;
  - it has the RB_NFIND macro which looks up the least upper bound of a sample
    node.

  I made some adjustments on both features.
  - regarding the static functions: I made the code a bit more portable;
  - regarding RB_NFIND: I replaced the FreeBSD implementation with a more
    streamlined and optimized one.