Creating extensions for LibreOffice Calc

Hi there

Is there any source of information specifically for developing Calc 
extensions to LibreOffice? I found the following page, but nothing 
relating to LibreOffice. Can we start something new on the wiki, perhaps?
http://wiki.openoffice.org/wiki/Extensions_development

Is there a pure C API for LibreOffice extensions, so that we can freely 
use any C/C++ compiler and not have to use one that is ABI compatible 
with the one used to compile LibreOffice? Or how does this work, noting 
comments given here:
http://wiki.openoffice.org/wiki/Extensions_best_practices#Programming_Language_.26_Environment

Is there any place where a trivial C-language Calc extension for 
LibreOffice is maintained, eg as part of regular unit testing work?

What would it take to adapt XLW (http://xlw.sfnet) so that it can build 
both Excel and LibreOffice extensions from unified source code, at least 
for very simple Calc extensions? The project I'm looking to port to 
LibreOffice is freesteam, which calculates steam properties for 
different pressure, temperature, etc: http://freesteam.sourceforge.net.

I will happily document your kind replies in the LibreOffice wiki.

Cheers
JP