remove -fno-optimize-sibling-calls flag for non-buggy setups

configure adds a `-fno-optimize-sibling-calls` flag, which is to work around a bug on gcc 3.3.3 on netbsd. I added a check to only add that flag if it's necessary. Or we could just remove it altogether as gcc 3.3.3 is quite old already. This is my first try at editing configure scripts so there may be a better way.