[plt-scheme] using sgl from mzscheme

I've got a glut-based C application that uses mzscheme as a scripting
language.  The C code's actually quite small, and I'd like to invert
the organisation and make the C code a loadable extension.  I can
readily build it as a loadable extension, and it loads fine, but under
mzscheme, I can't require sgl (get "undefined symbol"s for the GL
calls) and under mred, I get startup crashes in glut.  I presume that
mzscheme is a better way to go since I'm not using any mred bits at
all, so is there something I need to tweak in the sgl build scripts to
fix this? (I'm already using a bespoke sgl to get access to some GL
extensions).

John