Fix libbacktrace -fPIC breakage from "Use libbacktrace in libgfortran"

[Hans-Peter Nilsson]

> From: Ulrich Weigand <uweigand@de.ibm.com>
> Date: Wed, 26 Aug 2015 13:45:35 +0200

> Hans-Peter Nilsson wrote:
> > > From: Ulrich Weigand <uweigand@de.ibm.com>
> > > Date: Tue, 25 Aug 2015 19:45:06 +0200
> > 
> > > However, neither works for the SPU, because in both cases libtool
> > > will only do the test whether the target supports the -fPIC option.
> > > It will not test whether the target supports dynamic libraries.
> > > 
> > > [ It will do that test; and default to --disable-shared on SPU.
> > > That is a no-op for libbacktrace however, since it calls LT_INIT
> > > with the disable-shared option anyway.
> > 
> > Maybe it shouldn't?
> 
> Huh?  We do want libbacktrace solely as static library, that's the
> whole point ...

I meant that as a *suggestion for a possible workaround* to stop
libtool from refusing to compile with PIC, but then I take it
you don't need hints to try another angle than adjusting
compilation flags.

> > >  When adding back the -fPIC
> > > flag due to either the pic-only LT_INIT option or the -prefer-pic
> > > libtool command line option, it does not check for that again.  ]
> > 
> > Sounds like a bug somewhere, in libtool or its current use:
> > there *should* be a way to specify "I'd prefer PIC code in these
> > static libraries".
> 
> But that's what the option *does*.
> 
> Let me try again, maybe we can reduce confusion a bit :-)

I don't feel very confused, but I understand you've investigated
things down to a point where we can conclude that libtool can't
do what SPU needs without also at least fiddling with
compilation options.

> I guess we can always fall back to just hard-coding SPU once
> more; that's certainly the simplest solution right now.

Maybe.

brgds, H-P