Fixed having two ways of dealing with tcmalloc

Rationalizes the competing MEMORY_ALLOC_LIBRARY, TCMALLOC_ENABLED and MALLOC_LIBRARY variables.  Now MALLOC_LIBRARY doesn't exist any more:
* To use tcmalloc, set TCMALLOC_ENABLED=1
* To use a different memory allocation libary, use MEMORY_ALLOC_LIBRARY=mymalloc