mingw32 does not include function ffs.

Commit c6d29ad6 added a
declaration for ffs, but an implementation was missing.

For compilations with optimization, the compiler creates
inline code, so the implementation is not always needed.

Without optimization, linking fails without this patch.

v2: Use __builtin_ffs as suggested by Richard Henderson

Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>