Add GPL compatibility check for plugins

[Joseph S. Myers]

On Thu, 18 Jun 2009, Diego Novillo wrote:

> On Thu, Jun 18, 2009 at 16:11, Nathan Froyd<froydnj@codesourcery.com> wrote:
> 
> > Can you add a test that checks that if the symbol is not present, the
> > compilation fails?
> 
> Well, I've already checked that none of the existing tests work if
> they don't define that symbol.  I figured that was enough.

The point is that the state of the testsuite after this patch should 
verify this feature doesn't get accidentally broken in future.  The state 
of the testsuite before the patch may demonstrate that the feature worked 
when it was added, but does nothing to ensure it stays working.

-- 
Joseph S. Myers
joseph@codesourcery.com