[racket-dev] Mac OS X default build: 64-bit for 10.6, 10.7

I've changed `configure' so that 64-bit mode is no longer disabled by
default for Mac OS X builds. The build now uses the default mode of the
compiler, which is typically 64-bit mode for Mac OS X 10.6 and 10.7.

To get a 32-bit mode build when your compiler's default is 64-bit mode,
configure with `--disable-mac64'.

I know of no particular benefit to 64-bit mode for most programs, which
use much less than 4GB of memory, and 32-bit mode is usually 10% faster
(or as much as 30% faster for GC-intensive tasks). Since Racket now
works fine for 64-bit Mac OS X, however, it makes sense to switch the
default build mode to the system-wide default.