GCC's build fails in `libisl`'s configure step if `CC` is set to
Homebrew Clang with the message "Link Time Optimisation is not
supported". This is likely due to the fact that it tries to use ranlib
from Xcode, which is not compatible with the newer LLVM version's
bitcode format.
The toolchain build runs after `pick_host_compiler` is called, which
selects Homebrew Clang if the installed Xcode version is too old. We
need to unset `CC` and `CXX` for the toolchain build to sidestep the
issue.
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