Add g++ to required tools

Because gmp is now always build with --enable-cxx,
g++ has become a required tool

Signed-off-by: Bart vdr Meulen <bartvdrmeulen@gmail.com>

---

 # Compute IA-64-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

 }