Building _www-client/firefox_ fails fairly early with an error about not
being able to find `aarch64-unknown-linux-gnu-clang-19` to use as `CC`.
I have not been able to determine what is supposed to provide this
program/symlink, nor much information at all about cross-compiling with
Clang, really. We shall try building Firefox with GCC, since we know
that toolchain is complete.