Use #if rather than #ifdef for a potentially 0 value.
Use #if rather than #ifdef for a potentially 0 value.

We use 0 rather than leaving the macro undefined.  Identified in issue #54.

PiperOrigin-RevId: 337224855
Change-Id: I3782bba0a93b297a1c332d4e1de44a3f294562f0

google/tcmalloc,google/tcmalloc,google/tcmalloc