GCC: do not install GMP used to compile GCC

Other externals depend on their own GMP since we do not want to bind
the lifetime of GMP upgrades to the lifetime of GCC ones. This was done
to avoid reproducibility issues in e.g. FastJet and CGal.

gcc-toolchain.sh

@@ -121,7 +121,6 @@ pushd build-gcc
                    --disable-nls
   make ${JOBS+-j $JOBS} bootstrap-lean MAKEINFO=":"
   make install MAKEINFO=":"
-  (if cd gmp || cd ../gmp; then make install MAKEINFO=":"; fi)
   hash -r
 
   # GCC creates c++, but not cc