[plt-scheme] `make install' and DESTDIR

Eli Barzilay wrote:
> Alternatively, I asked for suggestions for some way of communicating
> such information to the make process, so it can create the right file.
> The only thing I can think of is environment variables for the extra
> paths, but that will be awkward and difficult to use (I think).

Seems simple enough to add an extra configure option to add system
collection paths to the build, since it's still way cleaner than having to
provide wrapper scripts that set PLTCOLLECTS.