[plt-scheme] The PLT/Boehm GC

I attempted to build mzscheme 207 on an HP/UX machine and I'm getting some compile error.  I was surprised by this as Boehm works for HP and does research with the collector involving gcj on HP/UX, so 

I downloaded the most recent GC release from Hans Boehm's site:

http://www.hpl.hp.com/personal/Hans_Boehm/gc/

and found that it does compile cleanly on HP/UX 11.  Then, I found that the gc included with mzscheme has a number of modifications not in the latest versions of Hans Boehm's gc.

Does anyone know if the mzscheme-specific modifications are required for the functioning of the mzscheme system, or , are they mostly performance related? 

The compilation problem with the gc included with the plt distribution is probably easy to overcome, but I'm wondering if there's not a better option.

Thanks.

-Jordan Henderson