If gettimeofday() is available we can achieve better than millisecond
precision (even if it usually isn't as high as microsecond), so use it as
clock source in wxStopWatch.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69841 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775