change CFLAGS for 64 bit icc builds

AMD64 only implies SSE2, not SSE3. There aren't any known cases where
icc was generating SSE3 instructions since all the vectorizable code
is already in handwritten asm, so this fix is included mostly for
correctness. Fixes issue #259.

Change-Id: I993335a4740b68b559035305fb52ca725a6beaff