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

Contributors :
==============

Leonid Tochinski ltochinski _at_ chattenassociates.com ( native win32 port (visual studio))

Vladimir Chren (kernel space port)

Raphael Zulliger (author of slavelib project)

Other unsorted contribs:
	Camille BOSSARD
	David DUMINY (sté A6R)
	Laurent ROMIEUX
	Zakaria BELAMRI
	Luis JIMENEZ

Many thanks to the main contributors for their great work.