packages.mk: update GCC prerequisites and fix build process

The GCC build process was broken on Centos 6.5 as reported by Sebastian
Leitmann.

Drop CLooG-PPL and PPL, which is not supported anymore by GCC. Use CLooG
and ISL from the GCC webpage instead. Copy GMP, MPFR, and MPC inside of
GCC source tree to avoid issues in detecting the prebuild libraries. (I
didn't do this for ISL and CLooG since GCC emits a warning in this case
about skipped version checks.)