Adding a compilation define so that on recent Darwin mysqld does not react to SIGHUP and SIGQUIT:
this fixes a rpl000001 problem on our Powermac G5 machine (popping after an upgrade from 10.3.4 to
10.3.5) and is expected to fix 
BUG#2030 "relay bin log on slave resets multiple times a second"
(i.e. under some Mac OS X Panther versions, mysqld receives many SIGHUP and SIGQUIT).
So this fix is more a problem-hider than a real understanding of why mysqld receives so many signals.
Note that we saw other problems on this OS where mysqld reacts to Ctrl-Z but apparently only once,
where using SSL seems to make the problem more frequent...