timers_unix: Fix termination problem of WaitReceiveTaskEnd

The function pthread_kill sends the Signal thread and to the own process.
If you use this construct than the application which calls uses the
canfestival api will terminate at the call of canClose. To avoid that
use pthread_cancel instead of pthread_kill. To use the pthread_cancel call
you need to set the cancel ability in the thread function. That means
you need to call pthread_setcancelstate and pthread_setcanceltype.
For the termination of the thread at any time it is important to set the
cancel type to PTHREAD_CANCEL_ASYNCHRONOUS.

some fixes

Fix : libcanfestival_unix.a not built if examples are not compiled

Problem: when building Canfestival with unix timers 'libcanfestiva_unix.a'
is built only if you build examples too.

Test case:

$ ./configure --timers=unix --can=socket --target=unix
$ make canfestival

...

no errors but 'libcanfestival_unix.a' is not built:
$ find . -name "*.a"
./src/libcanfestival.a

Changing line 90 of 'drivers/unix/Makefile.in' to:
#driver: $(OBJS)
driver: libcanfestival_$(TARGET).a

solves the problem:

$ ./configure --timers=unix --can=socket --target=unix
$ make canfestival

...

$ find . -name "*.a"
./drivers/unix/libcanfestival_unix.a
./src/libcanfestival.a