Add feature test macros

The strsignal function needs it.
Remove includes from public header
Reorder includes
Add time measuring using clock_gettime
Make timer smaller
Document the timer
Move include to .c file
Track whether timer is running

This should make it more difficult to use the module incorrectly. When
something fishy is going on, the timer prints warnings to stderr.
Do not crash if memory allocation fails
Exit child processes with success code
Remove check for libutest

It does not really make much sense to check for library that we are
building.