[MinGW-dvlpr] mingw symlink

[NightStrike <nig...@gm...>]

I tried building the toolchain for the first time in a long time, and
I see this is still an issue:

The directory that should contain system headers does not exist:
  /opt/mw64/sysroot/mingw/include

Is it at all possible to get rid of that dependency?  As I recall,
that was a holdover from mistakes made eons ago.  I get that "/mingw"
is the Windows equivalent of "/usr", but the issue here is that the
build system extends the sysroot variable.  It looks for
$sysroot/usr/include or $sysroot/mingw/include instead of just
$sysroot/include.

Does --with-build-sysroot make this go away?  Or, can it be made to do so?