Currently timer_getoverrun() only
reports timer notifications that are delayed in the timer
subsystem. If they are delayed in the signal subsystem, due to
signal masks for example, this is not counted as an overrun.
The option CYGPKG_POSIX_TIMERS allows the timer support to be
enabled or disabled, and causes _POSIX_TIMERS to be defined
appropriately. This will cause other parts of the POSIX system to
have limited functionality.