Substitute absolute path to files() when used in string formatting

From https://github.com/systemd/systemd/pull/5704#issuecomment-292796493:

But I have a trouble with makes files() work in more complicated cases: I need to a path to libudev.sym to specify it as a substring in -Wl,--version-script= option. I tried '-Wl,--version-script=' + libudev_sym '-Wl,--version-script=@0@'.format(libudev_sym)

In either of those two cases, we should substitute the absolute path to the files() object. I thought about relative paths, but there are too many corner-cases.