lib/macros.c: Use `void (*)(void)' as the universal function-pointer.

Newer versions of GCC complain about all kinds of explicit function-
pointer conversions unless (a) the arguments are /really/ similar,
or (b) they're to this specific `universal' function-pointer type.

The code which sets and uses these things looks safe to use, so I don't
think we need any further countermeasures.