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

[Desktop Entry]
Encoding=UTF-8
Name=CANOpen Object Dictionary Editor
Name[fr]=Editeur de Dictionnaire d'Objets CANOpen
Comment=Create, modifify, configure CANOpen nodes
Comment[fr]=Créer, modifier, configurer des noeuds CANOpen
Exec=/usr/bin/objdictedit
Icon=/usr/share/CanFestival-3/objdictgen/networkedit.png
Terminal=false
MultipleArgs=false
Type=Application
Categories=Application;Development;
StartupNotify=true