[m-dev.] for review: clean up handling of config macros

[Tyson Dowd]

On 15-Mar-1998, Fergus Henderson <fjh at cs.mu.OZ.AU> wrote:
> 
> /*
> ** MR_USE_STACK_LAYOUTS -- stack layouts are in use, generate stack
> **                         layout structures.
> */
> #if defined(MR_STACK_TRACE) || defined(NATIVE_GC)
>   #define MR_USE_STACK_LAYOUTS
> #else
>   #undef MR_USE_STACK_LAYOUTS
> #endif

I'm not sure about #undefing.  It should be possible to set flags using
-D and have them work -- won't this file would just undefine them?

Otherwise this file is fine.