Prevent cluttering gcov-options with local options

WIthout this patch the gcov is invoked on consecutive files as follows:
```
gcov --object-directory /home/ptomulik/projects/dimbo/build/lib/dimbo/test/unit/dimbo exception.gcno
gcov --object-directory /home/ptomulik/projects/dimbo/build/lib/dimbo/test/unit/dimbo --object-directory /home/ptomulik/projects/dimbo/build/lib/dimbo/test/unit/dimbo/cl device.gcno
gcov --object-directory /home/ptomulik/projects/dimbo/build/lib/dimbo/test/unit/dimbo --object-directory /home/ptomulik/projects/dimbo/build/lib/dimbo/test/unit/dimbo/cl --object-directory /home/ptomulik/projects/dimbo/build/lib/dimbo/test/unit/dimbo/cl devices.gcno
gcov --object-directory /home/ptomulik/projects/dimbo/build/lib/dimbo/test/unit/dimbo --object-directory /home/ptomulik/projects/dimbo/build/lib/dimbo/test/unit/dimbo/cl --object-directory /home/ptomulik/projects/dimbo/build/lib/dimbo/test/unit/dimbo/cl --object-directory /home/ptomulik/projects/dimbo/build/lib/dimbo/test/unit/dimbo/cl cl_errors.gcno
...
```

This patch prevents from multiplying --object-directory and other local flags.