Change optimization level to O2.

Fixes issue #9 - Reproducable crash

In a x86_64 build, gcc was turning the mem_bswap32 loop into 3 loops
(space-speed tradeoff) and doing 4 swaps at a time in the middle
loop with MOVDQA, which needs 16-byte aligned addresses.
It crashed because gcc assumed ptr is 4-byte aligned, when it was
actually 2-byte aligned.
Further changes might come later, when I understand why it made that
assumption.

It was also reported that using O2 makes packet delivery in general,
and frame relay especially more reliable.

Reported-by: Skylis@github.com