0 is not a power of 2

[w0rp via Digitalmars-d]

I believe you can also do x & -x == x. I'm not sure if that will 
be actually faster or slower. You could maybe cut the 
instructions down a little with an asm{} block. The compiler 
might not figure out that it can re-use a register for x on the 
left and x on the right there. You might use popcnt in a 
version() block too, so you can use the instruction when you've 
got it.