Set GC_DEBUG for debug builds

.. when building a debug build of GC with CONFIG += debug
   then we define a macros GC_DEBUG.

.. main no longer redirects to goldencheetah.log if you have
   GC_DEBUG set, so developers will continue to see debug
   messages on the terminal output (for mac/linux only).

.. we could use the GC_DEBUG macros to allow debug messages
   to be switched on / off in the code in future