[plt-scheme] embedded mzscheme calling extension

I've got an application which embeds mzscheme; it loads a script (via
scheme_require) which requires some extensions written in C.  This all
works fine on 209, but on >=300 the C extensions fail to load with
"undefined symbol: scheme_void".  The same extension loads with
problems from a REPL.

This is on i386 debian unstable, using a home-built version of mzscheme
(tried it on 301.7 and 301.9).

Any thoughts?  I'll prepare a minimal version if that's useful.

John