Add top-level Makefile for plugin feature

[awelzel]

I'm fairly used to build and install plugins via ./configure && make && make install, but the plugin feature doesn't seem to create a top-level Makefile.

Was that somehow on purpose or could we consider adding one?

[ckreibich]

We certainly could, Arne. We used to have such a Makefile, and it's still in the plugin skeleton here. I never warmed up to that built-it target, and figured that since the build will normally be hidden behind zkg's build_command, it's fine to let go. Feel free to put one together if you'd prefer it.

[0xxon]

FWIW - I am also in favor of this.