Hide the declaration of ffs() if an inline ffs() is implemented.
I couldn't find a better way to avoid compiler warnings about
redundant and/or inconsistent declaration of ffs(). I'd like to
be able to declare prototypes in general headers without committing
to implementing them as `static inline' or `extern', but there
seems to be no way to do this with gcc-2.6.1. E.g.,
int foo(void);
static __inline int foo(void) { return 1; }
causes a warning about the linkage mismatch, while the opposite
order causes a warning about the redundant declaration.