gcc with graphite: libppl.a needs libm

* workaround tested on ubuntu 13.04 x86_64 as a cross build.
buildtools: update gmp, mpfr, mpc libs

* gmp to 5.0.5, mpfr to 3.1.2, mpc to 1.0.1
PR bootstrap/50461

* configure.ac (mpfr-dir): When using in-tree MPFR sources
allow for the fact that from release v3.1.0 of MPFR the source
files were moved into a src sub-directory.
* configure: Regenerate.