update GMP for x86 and clang
Mac OS X: support Retina mode

Seems to work for Mac OS X 10.9 (Mavericks), at least.

In Retina mode, a drawing unit corresponds to two pixels on
the screen or in a bitmap created by `make-screen-bitmap'.
In particular, a bitmap created by `make-screen-bitmap' is
actually twice as big in each dimension as requested, and the
bitmap is scaled when transferring to other drawing contexts.
When transferring onto the screen, scalings cancel so that the
result looks right.

Adds `get-display-backing-scale` to `racket/gui/base`, and
also `get-backing-scale` to `bitmap%`.

To do: add a way to set the backing scale of a bitmap. That
option will provide a way to give controls higher-resolution
bitmaps as labels.