Fix getBit error.

[ndokmai]

I made the following changes:

• There seems to be a linking error with missing libpthread on Ubuntu 18.04 so I link libpthread in Makefile.in.
• getBit is reverted to the previous version with a slightly more elegant implementation.
• Instead of calling getBit(x, 65) and assigning it to xi, which is an off-by-one error, 0 is deliberately assigned to xi. (This, however, appears to be exhibiting the same behaviour as if you call getBit(x, 65) because apparently 1lu<<-1 == 0. But I think it's a good idea to keep this explicit.)
• fss-test.cpp is modified to make sure that it is correct with a=2.