Better Build Process for XS:Lisp (and v2?), including support in Makefile.user

[mesheets]

The XS:Lisp implementation does not currently afford much flexibility with file paths, making it a challenge to add flexible support to Makefile.user.

Some files—and even more than just *.h files—might benefit from being moved under the "include" folder, as otherwise either a relative or absolute path needs to be provided.

The xs-rcx and xs-host programs dump out a fixed-name "xsout" file in whatever happens to be the current working directory; this file then gets included in eval.c.

 
Possible version 2 of XS:Lisp?

• xslisp2: “Next generation of xslisp fused together with brickos for saving space, plus additional functionality”
• Initial integration into brickOS-bibo was based on XS:Lisp support built around brickOS 0.2.6.10; see the following for reference:
  • https://github.com/moyhig/xs_brickos-0.2.6.10 (“original from yuasa”)
  • https://github.com/moyhig/xs_brickos