Allows to build plugin as independent package

Hi,

I recently created a devel package for OpenCPN, so that plugins can be built separatly.
I patch yours, hope you'll enjoy it.

Please, see the [wiki page](https://en.opensuse.org/OpenCPN-build-plugins) and the [build page](https://build.opensuse.org/project/show/home:OrbitalRio:branches:Application:Geo:OpenCPN:head), where the OpenCPN-devel package is.