It's mostly not used anymore and doesn't do what it says it does.

The docs state that it affects GList, GSList, GNode, GMemChunks, GSignal,
GType n_preallocs and GBSearchArray while:

* GList, GSList and GNode use GSlice and are not affected
* GMemChunks is gone
* GType npreallocs is ignored

It also states that it can be used to force the usage of g_malloc/g_free,
which is handled by G_SLICE=always-malloc now.

The only places where it's used is in signal handling through GBSearchArray
and in GValueArray (deprecated). Since it's unlikely that anyone wants to
reduce allocation sizes just for those cases remove the build option.