threading: Get rid of warnings with old compilers
threading: Prevent possible division by zero
OFTimer: Reschedule before executing the callback

This makes sure that a repeating timer which e.g. needs half a second to
execute, but has an interval of 1 second, is indeed called every second
and not every 1.5 seconds.
Warn if configure.ac is newer than configure

This should drastically reduce the number of bug reports, as running an
old configure script with new sources (in other words, forgetting to run
./autogen.sh after git pull) seems to be the most common issue.