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
acdc783418.
