[plt-scheme] compiling PLT from source and ~/.plt-scheme

Eli,

> > SETUP_ARGS = -X "$(DESTDIR)$(collectsdir)" -l setup
> > 
> > Is there a reason why the "-U" option is omitted by default?
> 
> Yes, when you build your own code, you'd want to compile stuff in your
> own directory, or you'll get errors when loading it later.

What do you think about adding an option to the configure script 
to be able to use "-U" without patching Makefile.in?  This would
make external packaging easier.  I.e. when building a .deb you
really don't want the build process to go looking in your
~/.plt-scheme directory.  (For me it broke the .deb build because
I had some junk there..)