This came up when trying to build GHC HEAD with nix. We
do not set CCX for integer-gmp when running ./configure. We do
this however for libffi.
The result is, that if CCX is not set, we default to the system
one, of which there might be none (as in nixos's case). This
will not show on a debian+nix or similar setup, where the system
cxx is still in place, and only shows up when the system tries
hard to sandbox everything (even cxx) as nixOS does.
We use CXX, which is set to either clang or CC_STAGE1, and
also usedfor CC, similar to what we do for libffi.c