bug#15842: Integer overflow on Mac OS X 64-bit

[William S Fulton]

I verified the output of:

(display (- (expt 2 64) 1))(newline)
gives -1 using 1.8.8 on OSX Mountain Lion. I then installed Guile 2.0.9 
and the output is 18446744073709551615, so this looks fixed to me. The 
SWIG tests mentioned also pass now.

William