Fix fwrite$UNIX2003 symbols when cross-compiling for Tiger

- When compiling with -mmacos-deployment-target=10.4, we need 
  --no-builtin-fprintf, as the use of GCC's builtin function 
  optimisation for fprintf together with #include "PosixSource" in the 
  RTS leads to the use of fwrite$UNIX2003 (with GCC 4.0.1 on Mac OS X 
  10.5.2).