Avoid duplicate declarations, assumptions about HAVE_BIGNUM, fns.c
src/ChangeLog addition:
2011-01-02 Aidan Kehoe <kehoea@parhasard.net>
* fns.c (FdeleteX, FremoveX, Fnsubstitute, Fsubstitute, syms_of_fns):
Don't repeat the declaration and DEFSYMBOL() for Qnintersection in
this file; don't assume that bignums are always available. Fixes
some of the build problems the buildbot is showing me at the
moment.