Fix upgrading with GCC (blocker, #13395) needs review

[Jeroen Demeyer]

The GCC spkg broke upgrading from old Sage versions because of the
LD_LIBRARY_PATH not containing lib32 or lib64. We can fix this using
Leif's idea of sage-env versioning (see #10469).

We should also rebuild GCC if gcc was already installed before.

I have made this a sage-5.3 blocker because it breaks upgrading.

Please review:
http://trac.sagemath.org/sage_trac/ticket/13395

Thanks,
Jeroen.