[plt-scheme] trunk build failure, and help-desk failure

These two problems are probably unrelated, but they both showed up at
the same time, so I'm reporting them together.

First: I built trunk r7983, from an entirely clean build, on vanilla x86 linux.

I did "./configure && make -k all install"

I saw 

    setup-plt: Error during Compiling .zos for HtDP Teachpacks (/usr/local/src/plt-trunk/collects/htdp)
    setup-plt:   matrix.ss:57:17: compile: unbound variable in module in: cache-image-snip-class%
    make[1]: *** [install-3m] Error 1
    make[1]: Leaving directory `/usr/local/src/plt-trunk/src'
    make: *** [install] Error 2

    Compilation exited abnormally with code 2 at Wed Dec 12 21:38:11

Then, assuming that that problem was minor, I tried running help-desk:

    $ /usr/local/src/plt-trunk/bin/help-desk

That opened my web browser to the non-existent page 

    file:///usr/local/src/plt-trunk/doc/start/index.html

-- 
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