Use 'sig_atomic_t' when appropriate

Fixes #51.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Declare 'stop' as static in 'fast_orderbook'

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Merge pull request #54 from jvirtanen/fixes/signal-handler

Use 'sig_atomic_t' when appropriate