[patch][graphite] Fix configure for PPL installed in /usr

[Paolo Bonzini]

Sebastian Pop wrote:
> Hi,
> 
> I just realized that when ppl is installed in /usr/ gcc does
> not build as the ppl libs are not included in the ld line.
> Okay for trunk?  I'm testing the patch below on amd64-linux.

Okay, there's no way -lppl could get into the linker commandline
otherwise.  Note that a few dozen lines above we have

  # Check for GMP and MPFR
  gmplibs="-lmpfr -lgmp"
  gmpinc=
  have_gmp=no

Paolo