Path to setup.sh was hard coded in the script

This lead to a "Can't open /usr/local/share/gazebo/setup.sh" error when the installation is not made in the default path. This patch propose to use pkg-config to find the installation path. Command 

```bash
pkg-config --variable=prefix gazebo
```

return the installation path, adding share/gazebo gives the path to the setup.sh script.

Also there was a typo with the client_final variable initialization and a missing "$".