The dev-libs/libevent "clock-time" USE-flag and/or its description is very confusing and misleading.
# quse -D clock-gettime
dev-libs/libevent[clock-gettime] Disable use of clock_gettime even if it is available
So if one wants to disable clock_gettime, the correct way would be to enable the clock-gettime USE-flag, e.g. USE=clock-gettime ?
I recommend the USE-flag be renamed to include a prefix to denote negation, or to negate its semantics so that USE=clock-gettime would enable use of clock_gettime() instead of disabling it.