Enable -fuse-linker-plugin by default when possible

[Jan Hubicka]

> * Jan Hubicka wrote on Mon, Nov 29, 2010 at 02:18:52PM CET:
> > > On 11/29/2010 01:11 PM, Jan Hubicka wrote:
> > >> >  Can you instead add a dependency of configure-gcc on
> > >> >  configure-lto-plugin and use "test -f ../lto-plugin/Makefile" to detect
> > >> >  whether the plugin is in use?
> > >>
> > >> Hmm, OK, it seems bit hacky but easier than extra --with option...
> > >> Will do that.
> > >
> > > It is a bit hacky.  However, I'm starting to think that everything that  
> > > is added to the toplevel configure.ac is "very" hacky, so checking for  
> > > the Makefile is still better. :)
> > 
> > Indeed, updating autogened Makefile template and toplevel configury was no fun.
> > I spent couple days looking into this problem...
> 
> $ autoconf && autogen Makefile.def
> 
> or:
> $ .../configure --enable-maintainer-mode && make Makefile
> 
> That isn't too hard.

Yep, this is easy. My problem was to work out resonably clean way to get
the flags passed down to the GCC configure ;)
> 
> > I however run into new problem. With plugin enabled libtool breaks.
> 
> Upstream libtool has this fixed.

Really? I run into this with Mozilla and it seems to be quite recent, I will
check with upstream. Was direct calls to linker completely eliminated (this was
done for -flto, right?) or this particular grepping fixed?
I guess breaking older libtool with default configuration of GCC is still
something we should avoid, right?

Honza
> 
> Paolo, have you looked into the --with-sysroot issues that prevent us
> from updating the Libtool used in GCC currently?
> 
> Thanks,
> Ralf