POSIX is enabled.  This prevents the toolbox glue, all of Carbon,
and various other non-POSIX features from compiling.  The POSIX
symbols are  still used by default, so turning off the #define
doesn't hurt.

Additionally, linker flags have changed for Darwin 8, and are
different for Darwin 8/gcc4 (default) and Darwin 8/gcc3.3.

Approved by Anthony