[CMake] more on an executable dependency on external library

[Crni Gorac]

OK, I've tried in the meantime various arrangements with
ADD_CUSTOM_TARGET(), but seems hard to employ it to have CMake to put
following simple dependency of an executable "foo" on system library
"libbar.a" into generated Makefile:
foo: /usr/local/lib/libbar.a

There exists of course ADD_EXECUTABLE() statement for executable foo,
and there exists also TARGET_LINK_LIBRARIES() statement specifying
that "foo" should be linked with "libbar.a".  The library "libbar.a"
is a system-wide installed library, however let's say it changes
often, and it is necessary to have above dependency somehow captured
by CMake.  Any suggestion on how to accomplish this?

Thanks.