Fix collision with fls(int) on osx.

On OSX Mavericks at least, probably back to 10.5, fls(int) is defined in string.h.

I just prefixed the internal function fls with y, not sure if thats critical or not naming wise but I can rebase to a better name of whatever if you want.

Other than that it runs fine on osx no issues. This whole library is brilliant by the way I loathe arg parsing in general but this is quite nice indeed. Keep up the good work!