[CM] Cannot compile s7 on msvc

[bil at ccrma.Stanford.EDU]

> warning C4013: 'lookup' undefined; assuming extern returning int

I was trying to get rid of some #if's and forgot to include
the lookup forward declaration in the msvc case; here's
a change that ought to fix that problem:

at line 4061, add the declaration

static inline s7_pointer lookup(s7_scheme *sc, const s7_pointer symbol);

Thanks for the bug report!