Add GPL compatibility check for plugins

[Paolo Bonzini]

> Basically this means that if you want to use a non-GPL plugin, you have
> to modify your copy of gcc.  The FSF release of gcc won't run a non-GPL
> plugin, but of course nothing stops you from modifying the gcc sources.
> In other words, right #0 is not violated, since nothing stops you from
> doing whatever you want.

I don't think this is acceptable, and it goes totally against the spirit 
of the GPL exception that was designed for libgcc.  The point of plugins 
is that you do not have to modify GCC to use it, of course.  Besides, 
even though I'm sure Diego was in good faith, it should have been 
announced on the gcc@gcc.gnu.org mailing list by the SC rather than 
being buried in a patch whose subject is a white lie.

Note that it's just the technical implementation that I disagree with. 
If the FSF wants an error, fine.  But the bare acceptable minimum is to 
say "this copy of GCC won't run plugins that are not licensed under a 
GPL-compatible license", so that it's clear that it's legal to run 
non-GPL-compatible licenses; plus, make it a sorry rather than a 
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 hope binary distributions will rip out the warning (I'm speaking just 
for myself of course), but for now it should be clear that we're asking 
some of the people that develop plugins for e.g. academic purpose to do 
one of these:

1) lose time with the legal department of their university instead of 
hacking at GCC;

2) declare a GPL license that they do not have the right to declare;

3) go through more hoops than necessary to use a new feature;

4) use a svn checkout from before the warning was added (but with all 
the bugs that were fixed afterwards).

Paolo