i915g: Do not build winsys and binaries by default

i915g: Do not build winsys and binaries by default

Using a hack in the configure script the gallium intel
drivers have 3 options. Off, nothing is built. On, the
driver and binaries are built. Auto, only the driver but
not the binaries and winsys is built.

Since the i915g driver builds everywhere its can enable
the driver per default, so we can get build coverage.
But building the binaries per default is a pain for
distributions and testers since they conflict on the
install target with the old mesa drivers. Which are more
stable/faster/better.

So this change gives us the best of both worlds.