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

--- a/drivers/unix/Makefile.in	Sun Jun 08 17:42:47 2014 +0200
+++ b/drivers/unix/Makefile.in	Thu Jun 12 14:07:16 2014 +0200
@@ -87,7 +87,7 @@
 else
 CFLAGS = SUB_OPT_CFLAGS
 
-driver: $(OBJS)
+driver: libcanfestival_$(TARGET).a
 
 %.o: %.c
 	$(CC) $(CFLAGS) $(PROG_CFLAGS) ${PROGDEFINES} $(INCLUDES) -o $@ -c $<