[crossfire] Makefile fun

[Nicolas Weeger (Laposte)]

Hello.

I think the makefiles for the plugins are kind of weird :)

Currently the Python makefile references directly 
the ../common/plugin_common.c file, so the makefile for plugin_common is 
basically useless.

Meaning when we add files to the common directory we must update the Python 
makefiles, something I think is weird.

IMO, plugin_common should generate a library, which the Python plugin links to 
during build.

But I'm no makefile expert, so I don't want to break anything :)

Nicolas