This results in fewer conversion on common modern systems with
support for clock_gettime, pthread_cond_timedwait and ppoll.
gettimeofday is declared obsolete by POSIX.1-2008, so it is yet
another reason to move away from it.  This also appears to result
in the reduction of compatibility code required for dealing
with inconsistent implementations of "struct timeval".tv_sec

In the future, this will also result in fewer conversions for
kqueue and pselect if we elect to use them.

[ruby-core:85416] [Feature #14452]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62272 b2dd03c8-39d4-4d8f-98ff-823fe69b080e