Add GPL compatibility check for plugins

[Robert Dewar]

Paolo Bonzini wrote:

> fatal_error.  And to me, the ideal solution would be to add 
> -fnon-gpl-plugins that would suppress the sorry and add a note to the 
> output file saying "this file was compiled with a non-free plugin, so it 
> is legally under the GPL".

I don't like that phrasing: "under the GPL" is peculiar, what you really
mean to say "this file was compiled with a non-free plugin, so it can
only be distributed under a GPL license". It is only distribution that
is an issue!