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

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Contributors :

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==============

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

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Vladimir Chren (kernel space port)

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Raphael Zulliger (author of slavelib project)

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Other unsorted contribs:

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	Camille BOSSARD

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	David DUMINY (sté A6R)

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	Laurent ROMIEUX

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	Zakaria BELAMRI

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	Luis JIMENEZ

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Many thanks to the main contributors for their great work.