Fix btsync-makeconfig.sh referencing files before they are installed

These changes make `btsync-makeconfig.sh` work when building the package on a system where the package has not already been installed.

- Prepend `${srcdir}` to PATH in `build()` so that `btsync-makeconfig.sh` can find the `btsync` executable before it has been installed in `/usr/bin`
- Use relative path when referring to `btsync.conf.doc`