Update SConscripts to re-enable or add support for EGL on windows and
x11 platforms respectively.  targets/egl-gdi is replaced by
targets/egl-static, where "-static" means pipe drivers and state
trackers are linked to statically by egl_gallium, and egl_gallium is a
built-in driver of libEGL.  There is no more egl_gallium.dll on Windows.