gcc's configure system has the nasty habit (for us) of judging for
itself if it is building a cross compiler (or cross compiling), but on
the limited information of the build, host and target platforms' config
which only contains a subset of the information we encode in
`stdenv.*Platform`. The practical consequence was that prior to this
change building `pkgsLLVM.buildPackages.gcc` actually fails because it
refuses to use `--with-headers` with something it believes to not be a
cross compiler.
As a workaround we force the appropriate variable in the configure
script to always be `yes` regardless of its own conditional check.
At some point we probably should report this issue in some capacity, so
future gcc versions don't force us into workarounds like this and
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